The Sources in Detail | Betrayal as Strategy: Kobayakawa Hideaki and the Game Theory of Sekigahara

Evidence C-1How the Sources Are Handled

Evidence C-1 sorts the material this book uses into eight categories: contemporary sources, later house genealogies and memoranda, war chronicles, modern scholarship, received views, the author's own inferences, assumptions made for the analysis, and outright fiction. It also describes the nature of the principal sources and studies, the names "Eastern Army" and "Western Army," and what each topic assumes. The sources it rests on are these.

The copy of the jointly signed letter from Ishikawa Yasumichi and Hikosaka Motomasa to Matsudaira Ienori, dated the 17th day of the 9th month of Keicho 5 (1600), is no. 5 of the Hori monjo [Hori documents], held by the Archives (Kiroku Shiryokan) attached to Kyushu University Library. It is not the original but a copy, and it bears a colophon (okugaki) by Honda Masanobu dated the 26th day of the 9th month (transcribed in Shinshu Fukuoka shishi [New History of Fukuoka City], shiryo-hen, Chusei 1, pp. 177–178, according to note 183 of part 3 of Shiramine Jun, "Keicho 5-nen 6-gatsu – donen 9-gatsu ni okeru Tokugawa Ieyasu no gunji kodo ni tsuite"). It was written two days after the battle, by two men in the Sawayama area who were in a position to relay the course of the fighting to the rear. This book calls it "the copy of the jointly signed letter of the 17th day of the 9th month," and discusses Ieyasu's arrival at Akasaka in Evidence C-4, the Western Army's withdrawal from Ogaki and the time of the main battle in Evidence C-5, and the daimyo who switched sides and the commanders who were killed in Evidence C-6.
The letter from Tokugawa Ieyasu to Date Masamune dated the 15th day of the 9th month of Keicho 5 was written by Ieyasu himself on the day of the battle, and calls the battlefield "Yamanaka in Noshu" (Evidence C-5).
The memorandum (oboegaki) of Miyanohara Saibei, a retainer of Shimazu Tadatoyo (colophon dated Kanbun 3), the memorandum of Inoue Shuzen (colophon dated Meireki 3) and a memorandum by another retainer (colophon dated Kanbun 4), both of the Shimazu house, and the later account by Mizuno Katsunari, who was with the Eastern Army, all report on Ogaki on the night of the 14th day of the 9th month, and all are later memoranda written by participants or their retainers. Although they are not contemporary documents, they are treated differently from war chronicles in that they survive independently in more than one house. Miyanohara Saibei's text is sometimes introduced as a "report" (kakiage), but the published edition (Kagoshima-ken shiryo: Kyuki zatsuroku shui, Kirokusho shiryo 1) titles it a memorandum (oboegaki). Where each is published, and what each says, is set out in Evidence C-5.
The Kan'ei shoka keizuden [Genealogies of the Various Houses, Kan'ei Era] and the Kansei choshu shokafu [Revised Genealogies of the Various Houses, Kansei Era] are genealogies compiled by the Tokugawa shogunate, and also sources that tend to emphasize contributions to the Tokugawa side. The house entries and the books and parts relied on are given item by item in Evidence C-2 to C-6.
Sekigahara gunki taisei [Compendium of the Sekigahara War Chronicles] is a war chronicle completed in Shotoku 3 (1713). Among the later accounts this book mentions, it is the source for the terms the Western Army reportedly offered and for the written oath of the 14th day of the 9th month to Hiraoka Yorikatsu and Inaba Masanari (Evidence C-3), the circumstances of Mitsunari's capture (Evidence A-5), and the anecdote of the 600 under Otani's direct command pushing back the Kobayakawa forces (Evidence C-6).
The Army General Staff's Nihon senshi: Sekigahara eki [Japanese Military History: The Sekigahara Campaign] (Meiji 26, 1893) is a military history of the Battle of Sekigahara compiled by the general staff of the army. It has a narrative text and deployment maps, and together with Kamiya Doichi's Sekigahara kassen zushi [Illustrated Record of the Battle of Sekigahara] (Meiji 25, 1892) it is said to have been the basis of the positions in today's received view (Evidence A-9 and Evidence C-6). The nature of the figures in its appended table of troop numbers is explained in Evidence A-6. The documents behind it are collected in Army General Staff, ed., Nihon senshi: Sekigahara eki, monjo [Japanese Military History: The Sekigahara Campaign, Documents] (Genshinsha, 1911).
The published locations and dates of the letters cited in Evidence C-3 to C-5 on Ieyasu's whereabouts in the 8th month, Mitsunari's stay in Ogaki Castle, the fall of Gifu Castle, the standoff at Akasaka, and Ieyasu's arrival at Gifu and at Akasaka are those given in Shiramine Jun, "Keicho 5-nen 6-gatsu – donen 9-gatsu ni okeru Tokugawa Ieyasu no gunji kodo ni tsuite" [On Tokugawa Ieyasu's Military Operations from the Sixth to the Ninth Month of Keicho 5 (1600)] (part 1: Beppu Daigaku kiyo 53, pp. 65–77; part 2: Beppu Daigaku Daigakuin kiyo 14, pp. 37–56; part 3: Beppu Daigaku Shigaku Kenkyukai, Shigaku ronso 42, pp. 1–33; all 2012).
The designations "Gotairo" (Five Elders) and "Gobugyo" (Five Commissioners) have themselves been re-examined on the basis of the sources in Horikoshi Yuichi, "Toyotomi 'Gotairo' Gobugyo ni tsuite no saikento: sono kosho ni kanshite" (Nihon rekishi 659, pp. 19–37, 2003). Horikoshi later argued, in "Chigyo ategaijo ni miru Toyotomi 'Gotairo' no seikaku" [The Character of the Toyotomi "Five Elders" as Seen in Their Grants of Fief] (Kokugakuin Daigaku kiyo 48, pp. 341–361, 2010; later in Toyotomi seiken no kenryoku kozo, Yoshikawa Kobunkan, 2016), that in matters of fief grants the real power of decision lay with the "Five Commissioners." This book cites that point from Tani Tetsuya, "Shohyo to shokai: Horikoshi Yuichi cho Toyotomi seiken no kenryoku kozo" [Review of Horikoshi Yuichi, The Power Structure of the Toyotomi Regime] (Kokushigaku 223, pp. 228–233, 2017) (Evidence A-6, Evidence C-8).
"Eastern Army" and "Western Army" are not names that were used at the time of Sekigahara; in contemporary sources the sides are called by names such as "Terumoto's side" (Terumoto-gata) and "the commissioners' side" (bugyo-gata). Shiramine Jun uses "the allied army of Ishida Mitsunari and Mori Terumoto" and "the army led by Tokugawa Ieyasu" as designations closer to the realities of the time. That the names "Eastern Army" and "Western Army" first appear in the war chronicles in the Ishida gunki, completed in Genroku 11 (1698), comes from Shiramine Jun, "Sekigahara no tatakai no toraekata ni tsuite" [On How to Understand the Battle of Sekigahara] (Beppu Daigaku Shigaku Kenkyukai, Shigaku ronso 43, 2013), p. 51.
The account that after Hideyoshi's death the Five Elders and the Five Commissioners ran the government, and that after the death of Maeda Toshiie the balance broke down and Ieyasu came to take the lead, follows the headnotes (kobun) in vol. 13 of the Historiographical Institute of the University of Tokyo, ed., Shiryo soran [Chronological Summary of Historical Sources] (1954), and the Army General Staff's Nihon senshi: Sekigahara eki. That Ieyasu moved into Fushimi Castle on the 13th day of the intercalary 3rd month of Keicho 4 (1599) can be confirmed in the entry for that day in Tokitsune kyoki.
The figure of 2,420,000 koku for Ieyasu's holdings in the Kanto is the one Shiramine Jun gives, on the basis of a table of Toyotomi daimyo in a reference dictionary, in "Sekigahara no tatakai ni okeru shosho no gun'yaku ninzu (heiryokusu) ni tsuite no yosatsuteki kosatsu" [A Preliminary Study of the Military Levies (Troop Strengths) of the Commanders at the Battle of Sekigahara] (Kokushi sanshu 27, pp. 43–77, 2025). It cannot be traced in a contemporary register of koku assessments.
That Mori Terumoto was in the western enclosure (Nishinomaru) of Osaka Castle from the 17th day of the 7th month follows Shiramine Jun's study of Tokugawa Ieyasu's military operations (part 3).

Evidence C-2Fushimi Castle

For the siege of Fushimi Castle, contemporary sources survive in comparatively good number, and it can be followed day by day. When Hideaki arrived, and in what capacity he joined, differ from record to record.

The defense of Fushimi Castle by Torii Mototada and others began on the 15th day of the 7th month of Keicho 5 (1600); this comes from the entry for that day in Shunkyuki [Diary of Bonshun]. As for the number of defenders, a letter from Ishida Mitsunari to Satake Yoshinobu dated the 10th day of the 8th month of Keicho 5 gives 2,300: the castle garrison of 1,800 plus 500 who had moved over from the western enclosure (Nishinomaru) of Osaka Castle. That full-scale fighting began on the 19th and that the Ukita forces joined on the 22nd come from the entries for those days in Tokitsune kyoki; that the besiegers built mounds, set heavy guns and cannon on them, and filled in the moats, and still the castle did not fall, comes from document no. 626 in Kikkawa-ke monjo [Kikkawa family documents]. That the castle fell at midday on the 1st day of the 8th month appears in the entry for that day in Gien jugo nikki [Diary of Gien]; that Torii Mototada was killed by Suzuki Magosaburo (Shigetomo), a captain of musketeers, appears in a letter from Ishida Mitsunari to Sanada Masayuki and others dated the 5th day of the 8th month of the same year; and that 800 men, including Naito Ienaga and his son and Matsudaira Ietada, died fighting appears in a jointly signed letter from Mashita Nagamori and Natsuka Masaie to Sanada Masayuki dated the 1st day of the 8th month of the same year. An entry in Gien jugo nikki for the 21st day of the 7th month also records people from the neighboring villages watching from the hills at Ogurisu. That letters of commendation were sent round by the commissioners after the castle fell appears in a letter from Mashita Nagamori dated the 1st day of the 8th month and in a jointly signed letter from Natsuka Masaie, Mashita Nagamori and Maeda Gen'i dated the 2nd day of the 8th month; both are addressed to Nabeshima Shinano-no-kami and Mori Buzen-no-kami, and praise their feat in forcing their way into the main enclosure.
In the 7th month of Keicho 5, Kobayakawa Hideaki joined the siege of Fushimi Castle on the Western Army's side. The entry in Gien jugo nikki for the 23rd day of the 7th month has been cited in published introductions as saying that Hideaki had arrived the previous day as one of the attacking contingents, and this is why this book gives his arrival as the 22nd day of the 7th month. Mitsunari Junji, for his part, dates Hideaki's participation from around the 21st day of the 7th month.
That, while Fushimi Castle held out, Ieyasu was in the eastern provinces pursuing matters with the commanders by letter can be confirmed from the arrangement of his letters in Shiramine Jun, "Keicho 5-nen 6-gatsu – donen 9-gatsu ni okeru Tokugawa Ieyasu no gunji kodo ni tsuite" (part 1).
Where Hideaki stood in the siege of Fushimi Castle differs from record to record. Katsushige-ko fu koho [Supplemented Chronicle of Lord Katsushige] (Saga-ken kinsei shiryo, part 1, vol. 2) says of the army that set out for Fushimi late in the 7th month that "the commander was Kingo (Kobayakawa) Hideaki," names Shimazu Yoshihiro and Tadatsune, Ryuzoji Takafusa, Nabeshima Katsushige, Mori Yoshimasa and others as those who followed him, and puts its officers and men at more than 23,000. In its account of the castle's fall, too, it writes that "fire arrows were loosed from Kobayakawa Hideaki's own troops, and burned down the keep of Fushimi Castle." This is a house genealogy compiled by the Nabeshima house in the Edo period, and also the record of a house that was placed under Hideaki, so it must be treated with some caution. On dates it conflicts with the contemporary records, putting the start of the attack on the night of the 25th day of the 7th month, for example. Yet it gives both the man who killed Mototada and the more than 800 dead just as the contemporary records do, so being a later house genealogy does not make its contents empty. On the other hand, one account makes Ukita Hideie the supreme commander. This book follows Katsushige-ko fu koho and presents Hideaki as the commander. The Ukita forces had arrived on the 22nd day of the 7th month, and if what Hideaki led is read as a separate large army that joined on the same day, the two are logically compatible. There is, however, no direct source that allows this reading. This book does not say who put Hideaki in command.
When Hideaki set out from Chikuzen for the Kamigata region, and what he came for, are not known. There is a gap between his restoration to Chikuzen and Chikugo on the 5th day of the 2nd month of Keicho 4 and the siege of Fushimi Castle in the 7th month of Keicho 5. This book likewise begins with him already in camp at Fushimi and does not write how he came to be there.
This book gives no figure for the whole Western Army force that surrounded Fushimi Castle, because there is no figure that can be traced. The more than 23,000 used in Chapter 1 is a figure that goes back to Katsushige-ko fu koho; it belongs to the army that joined under Hideaki late in the 7th month, and is not the whole besieging army.
No contemporary source has been found that states what Torii Mototada was aiming at in holding the castle. The records listed above give dates, numbers, the course of the fighting and the dead, and say nothing of Mototada's intentions or Ieyasu's. No document issued by Mototada himself during the siege could be confirmed either. The account of how Ieyasu entrusted Fushimi Castle to Mototada and the others before leaving for Aizu, with the reasoning that this was a provision against Mitsunari's disloyalty, is in the Kansei choshu shokafu (completed in Bunka 9, 1812); the story that Ieyasu and Mototada exchanged a parting cup is in the Mikawa monogatari (Genna 8, 1622, by Okubo Tadataka). The first is a genealogy compiled by the Tokugawa shogunate; the second is a memoir written twenty-two years after the siege by a bannerman who was not there. The account that the Koka men inside the castle were won over and set it alight likewise cannot be traced to a contemporary source. The assessment of Mototada's stand as time bought for Ieyasu has been handed down to the present day, but the part of it that concerns his intention goes back no further than these later compilations.
There is a view that the Western Army's advance into Mino and Ise was delayed because it spent more than ten days at Fushimi. What the sources can show is the order of events. The castle fell on the 1st day of the 8th month (Gien jugo nikki, same day); Mitsunari wrote to Sanada Masayuki, Nobuyuki and Nobushige about the next arrangements on the 5th day of the 8th month (Sanada-ke monjo no. 54, transcribed in Abe Tetsuto, "Uesugi Kagekatsu in the campaign of Keicho 5"); and that Mitsunari was in Mino and Natsuka Masaie in Ise is confirmed by Mitsunari's letter of the 10th day of the 8th month to Sanada Masayuki and Nobushige (Shinano shiryo vol. 18, p. 454). The Western Army's deployment into Ise and Mino therefore comes after Fushimi fell. That Ieyasu was meanwhile advancing his talks with the commanders in the eastern provinces by letter can be confirmed from Shiramine Jun's survey cited above. Whether the Western Army could have moved sooner had Fushimi fallen earlier is something that did not happen, and the sources do not yield it.
The "thirteen days" that Hideaki counts in Chapter 1 is the number of days from the 19th day of the 7th month, when the attack became full-scale, to the fall of the castle on the 1st day of the 8th month; counted from the 15th day of the 7th month, when the defenders shut themselves in, it would be seventeen days. Since Hideaki arrived on the 22nd, this is not the number of days he saw on the spot but the number the besiegers as a whole spent. There is no source showing how Hideaki himself took it at the time (for the author's inference, see Evidence C-8).
There is a story that Hideaki, intending to side with the Eastern Army, tried to enter Fushimi Castle, was refused, and had no choice but to join the attackers. The same story is told of Shimazu Yoshihiro. The former rests on Kansei choshu shokafu and the latter on the "Shimazu kafu" [Shimazu House Genealogy], both house genealogies compiled in the Edo period, and neither can be confirmed in contemporary sources. This book does not adopt it.

Evidence C-3Negotiations with the Eastern Army Before the Battle

That the Eastern Army made approaches to Hideaki can be confirmed from a jointly signed letter dated the 28th day of the 8th month of Keicho 5 (1600). Who was involved in the negotiations, and how the Kobayakawa forces moved after Fushimi, rest on researchers' accounts, and there is no source showing when Hideaki decided to side with the Eastern Army (for the author's inference, see Evidence C-8).

Ieyasu returned from Oyama to Edo on the 5th day of the 8th month, and did not leave Edo during the 8th month until he set out on the 1st day of the 9th month. His return to Edo comes from a letter from Tokugawa Ieyasu to Date Masamune dated the 7th day of the 8th month, and his departure from a letter from Tokugawa Ieyasu to Fukushima Masanori dated the 1st day of the 9th month (Nakamura Koya, Tokugawa Ieyasu monjo no kenkyu [Studies of the Documents of Tokugawa Ieyasu], vol. 2, pp. 561–562 and 655).
A jointly signed letter from Kuroda Nagamasa and Asano Yoshinaga to Kobayakawa Hideaki, dated the 28th day of the 8th month of Keicho 5, survives. It is among the Izumo Kuwabara documents, formerly held by Kuwabara Yojiro, and it is written on a kogirigami, the small slip of paper used for secret letters. Invoking their ties to Kita no Mandokoro, it asks for his loyal service now ("kono tabi go-chusetsu kan'yo"), and says that Ieyasu will arrive within two or three days. It has no words saying "join the Eastern Army," but it reads as a letter urging him to side with the East. This book reads the form of the approach itself, a joint letter from Kuroda and Asano invoking Kita no Mandokoro's ties rather than a letter from Ieyasu himself, as the invitation. That Hideaki received this letter, or that he responded to it, cannot be said directly from the letter.
The letter says that they had written to him before ("sensho ni moshiire soro to iedomo"). That earlier letter does not survive, and neither its contents nor whether Hideaki replied are known. Of the approaches whose contents can be checked, the letter of the 28th day of the 8th month is the earliest. The Army General Staff's Nihon senshi: Sekigahara eki has it that after Fushimi Castle fell, it was Hideaki's side that sent a letter to Kuroda Nagamasa. Kuroda Nagamasa, in his testamentary memorandum (yuigon oboe) of Genna 9 (1623) (Kuroda-ke monjo [Kuroda family documents]), left it on record that winning over the Mori and Kobayakawa houses had been his own achievement. Kobayakawa Hideaki is said to have been in secret contact with the Eastern Army from before the battle, and Hiraoka Yorikatsu is said to be the man who, together with Inaba Masanari, handled the negotiations with Kuroda Nagamasa and led Hideaki to the Eastern Army. It is Nihon senshi: Sekigahara eki that makes Hiraoka's marriage ties with the Kuroda house the connection behind the negotiations, and that work gives neither its source nor between whom the tie existed.
The movements of the Kobayakawa forces after Fushimi Castle fell are unclear in the sources, and accounts differ. Kuroda Motoki, Kobayakawa Hideaki (Ebisu Kosho Shuppan, Shirizu: Jitsuzo ni semaru 005, 2017), pp. 61–64, holds that he did not join the army sent to take the Ise area but stayed encamped in Kyoto and Omi, alone in having left the front. Beyond this, some records have him advancing toward Ise, one theory has him assigned to guard against the Hokuriku direction, to the north, and another account has him staying a long time in Omi. This book adopts Kuroda's account, and presents him as not having gone on acting with the main Western Army as he had at Fushimi. There is not enough material to settle this, however, and there is no source showing what Hideaki himself intended by it. The entries for Kobayakawa Hideaki (book 618) and for Inaba Masanari (Kokumin Tosho edition, part 4, book 608) in the Kansei choshu shokafu state that, sent by Mitsunari toward Tsu Castle in Ise, Hideaki turned back at Seki no Jizo on the way and set up camp at Takamiya in Omi. This is an Edo-period house genealogy, and it points the same way as Kuroda's account that he did not join the army sent against Ise.
On the 3rd day of the 9th month, at Odawara, an envoy from Hideaki came to Ieyasu's retainer Nagai Naokatsu, and Ieyasu said that "what the young man says is not sincere; there is no need to deal with it," and did not take it up. An envoy who came on the 8th, at Shirasuka, did not appear before Ieyasu but received "a kind reply." This comes from the middle volume of Keicho nenchu Bokusai ki [Bokusai's Record of the Keicho Years] (Shiseki shuran 26). It is a later record by Itasaka Bokusai, who accompanied Ieyasu, and cannot be confirmed from contemporary sources. What the envoy conveyed is not recorded.
The entry for Kuroda Nagamasa in the Kansei choshu shokafu (Kokumin Tosho edition, part 3, book 425) states that Nagamasa sent a retainer to Hideaki's camp, and that Hiraoka Yorikatsu's younger brother Sukeshige was sent to Nagamasa as a hostage, and Nagamasa's retainer Yoshida Kunai to Hideaki. It does not give the date of the exchange of hostages. It also states that after Ieyasu reached Akasaka on the 14th day of the 9th month, Nagamasa, on Ieyasu's orders, sent an envoy and had Hideaki pledge that he would without fail turn against the Western Army. This is an Edo-period house genealogy, and of the tie between Hiraoka and the Kuroda it says only "because of a connection."
A written oath dated the 14th day of the 9th month from Honda Tadakatsu and Ii Naomasa to Hiraoka Yorikatsu and Inaba Masanari survives. Its three articles are that Ieyasu will not be remiss toward Hideaki; that if the two men render loyal service, Ieyasu will not be remiss toward them in future; and that "once your loyal service is settled," they will obtain for Hideaki a written grant of two provinces in the Kamigata. It thus promises the reward in the form of a condition. It survives as a copy in Sekigahara gunki taisei, and no original has been confirmed (Army General Staff, ed., Nihon senshi: Sekigahara eki, monjo [Japanese Military History: The Sekigahara Campaign, Documents], no. 108, p. 79). It is a different document from the written oath of the same date from the same two men to Kikkawa Hiroie (Evidence C-7).
Ieyasu's letter of the 24th day of the 9th month to the Chikuzen Chunagon says that "your loyal service at Sekigahara on this occasion is truly a matter of the greatest delight, and that there has been no departure from the course from the very first is a particular satisfaction." Ieyasu's letter of the 17th day of the 9th month to Inaba Masanari likewise credits Hideaki's loyal service to Inaba's resourcefulness. Both survive as copies in "Komonjoshu" (Army General Staff, ed., Nihon senshi: Sekigahara eki, monjo [Japanese Military History: The Sekigahara Campaign, Documents], no. 111, p. 82). They do not say from when, or what kind of understanding, "the course from the very first" refers to.
In the exchange of information in Keicho 5, what contemporary documents confirm is not the work of scouts or shinobi (covert agents) but letters passed through intermediaries. The letter of the 28th day of the 8th month above is written on a kogirigami, the small slip of paper used for secret letters, and both the choice of paper and the wording itself, which asks for loyal service by way of the tie to Kita no Mandokoro without naming the Eastern Army, show the conventions of the time. Under the date of the 14th day of the 9th month, Honda Tadakatsu and Ii Naomasa, and Fukushima Masanori and Kuroda Nagamasa, each issued written oaths (kishomon) to Kikkawa Hiroie (both in the Mori-ke monjo [Mori family documents]; Evidence C-7). On the very day Kobayakawa entered Mount Matsuo, then, the Eastern side was exchanging written oaths with a commander of the Mori forces on Mount Nangu.
The post-horse system (tenma), which kept men and horses stationed along the highways to pass messages on in relays, dates from Keicho 6 (1601) on the Tokaido and Keicho 7 (1602) on the Nakasendo. At the time of Sekigahara this system was not yet in place. The warlords and the Toyotomi regime had post-horse arrangements before that, but whether horses could be changed along this route at this time cannot be known from the sources. This book writes a fast rider as going on his own horse, or changing horses at a camp of his own house. That Gifu Castle had fallen, who had gathered at Ogaki, that the Eastern vanguard had come out to Akasaka: news of this kind spread through both camps within days, and is not the kind of thing whose source needs asking. That Ieyasu left Kiyosu on a given day, or entered Gifu on another, is what a decision to move that day rests on, and is worthless once it is late. Since changing horses along the way could not be counted on, a new movement far off has to be fetched by sending one’s own men.
Until the 14th day of the 9th month the Kobayakawa forces were treated as one of the Western Army's units.
No contemporary document has been found that records the reconnoitering of Mount Matsuo at Sekigahara. The Army General Staff's Nihon senshi: Sekigahara eki, p. 192, has a retainer of Takenaka Shigekado assigned to guide the envoy to Mount Matsuo. It is a case of using a local man as a guide, but this is a later record, and this book does not use that scene.
There is a story that the Western Army offered terms: the office of kanpaku (imperial regent) until Hideyori came of age, and two provinces in the Kamigata, the region around Kyoto and Osaka. It is Sekigahara gunki taisei that records this, and it has been pointed out that the document of terms it contains may be a forgery, since no original has been confirmed and its style also raises doubts. This book does not use these terms.

Evidence C-4Mount Matsuo

That Ieyasu was at Gifu on the 13th day of the 9th month of Keicho 5 (1600) and at Akasaka on the 14th can be confirmed from letters. Hideaki's whereabouts on those same days, and how he came to enter Mount Matsuo on the 14th, rest on researchers' reconstructions and on Edo-period genealogical records, and views differ.

Ieyasu arrived and set up camp at Gifu on the 13th day of the 9th month, and moved out to Akasaka on the 14th. This comes from a letter from Tokugawa Ieyasu to Hijikata Katsuhisa dated the 13th day of the 9th month (Nakamura Koya, Tokugawa Ieyasu monjo no kenkyu, vol. 2, p. 690) and a jointly signed letter from Nishio Yoshitsugu and Nishio Toshiuji to Niwa Nagashige dated the 14th day of the 9th month (ibid., p. 689; Gifu kenshi [History of Gifu Prefecture], shiryo-hen [source volumes], Kodai/Chusei [Ancient and Medieval] 4, pp. 204–205). The copy of the jointly signed letter from Ishikawa Yasumichi and Hikosaka Motomasa dated the 17th day of the 9th month (Evidence C-1) also says that he "arrived at Akasaka on the fourteenth just past."
Mount Matsuo Castle is said to be one of the largest mountain castles in western Mino, built by Ito Morimasa on the orders of Ishida Mitsunari (Nakai Hitoshi, "Sekigahara kassen to jokaku"). There is a view that the building work on this castle went so far as to envisage installing Mori Terumoto there; it was put forward by Nakai Hitoshi, from the side of castle studies. Nakai Hitoshi, "Sekigahara kassen to jokaku" [The Battle of Sekigahara and Castles] (in Gifu Sekigahara Battlefield Memorial Museum, ed., Sekigahara Kenkyukai kikoshu [Contributions to the Sekigahara Study Group], 2024, pp. 16–18), cites the passage in Mitsunari's letter of the 12th day of the 9th month (the item below) that puts the Chugoku forces into the castle of Matsuo on the border of Omi and Mino, and takes the castle to have been built to house the Chugoku forces, that is, Mori Terumoto.
Where Hideaki was around the 13th day of the 9th month is reconstructed variously as Takamiya or Kashiwabara. A prohibition placard (kinzeisatsu) of Hideaki's survives at Jobodai-in in Kashiwabara (designated a Cultural Property of Shiga Prefecture in Reiwa 6, 2024), and according to tradition he quartered there around the time of the battle, but the placard is dated only "9th month," and the date cannot be pinned down. His route from the 12th to the 14th day of the 9th month is a reconstruction following the received view, and re-examination from the primary sources has produced other views of where he was before the 14th. Takahashi Yosuke, "Kugatsu juyokka, seigun kara Kobayakawa Hideaki ni taisuru kosho" [The Western Army's Negotiations with Kobayakawa Hideaki on the 14th Day of the 9th Month] (in Gifu Sekigahara Battlefield Memorial Museum, ed. and pub., Sekigahara Kenkyukai kikoshu 2025 [Contributions to the Sekigahara Study Group, 2025], pp. 12–13, 2025) holds that the one at Ogaki on the 14th was Otani, and that Hideaki had moved from Kashiwabara to Yamanaka. The Toda Samon oboegaki [Memorandum of Toda Samon] (transcription in Chiikishi kenkyu 116, p. 172) states that Hideaki besieged Otsu Castle until the 13th and arrived at Sekigahara at the hour of the Monkey on the 14th. It is a later memorandum, and since it does not fit this book's route, this part of it is not used. This book presents Hideaki as having halted at Kashiwabara (that the arrangements at Takamiya and Kashiwabara are the author's inference is noted in Evidence C-8).
The received view is that on the 14th day of the 9th month Kobayakawa Hideaki made Ito Morimasa withdraw from Mount Matsuo Castle, entered it, and took up position on the mountain. Among recent accounts, Mitsunari Junji, Kobayakawa Takakage, Hideaki [Kobayakawa Takakage and Hideaki] (Minerva Shobo, 2019), p. 255, adopts it. No contemporary document can be confirmed, however, and the scene of Ito's withdrawal rests on Edo-period genealogical records such as Kan'ei shoka keizuden and Kansei choshu shokafu (Evidence C-1), so the date of the entry and the details of how it came about cannot be taken at face value. Within the Kansei choshu shokafu itself, the entry for Inaba Masanari says that he entered the new castle on Mount Matsuo (item below), while the entry for Kobayakawa Hideaki says that "on the 14th he drew up his forces at the foot of Mount Matsuo." Shiramine Jun judges it highly likely that, by the time the battle opened, Hideaki had moved down from the summit and taken up position on the low rise in front of Mount Matsuo (where the received deployment maps place Wakisaka and the others). This book adopts the received view that he entered at the summit. That he entered without an order from Mitsunari is the author's inference (Evidence C-8); Mitsunari Junji holds it highly probable that he entered as part of the Western Army's strategy.
No contemporary document ordering the entry survives from either side. The entry for Inaba Masanari in the Kansei choshu shokafu (Kokumin Tosho edition, part 4, book 608) states that, because the Western Army was plotting to attack Hideaki's camp, on the 14th day of the 9th month Masanari consulted the other retainers, led the troops into Mino, entered the new castle on Mount Matsuo, and drove out its commander, Ito Nagato no Kami. The account first appears in the Kan'ei shoka keizuden. Nakai Hitoshi, "Sekigahara kassen to jokaku" (the item above, p. 17), regards entering the castle after driving out its commander as "an unlawful seizure, not a Western Army operation," and as suggesting that Hideaki's army entered the castle as part of the Eastern Army from the start; the same article notes that until then Hideaki's deployment on Mount Matsuo had been thought to be part of Otani Yoshitsugu's dispositions. This book presents the entry as made on their own judgment, not on Mitsunari's orders (for the author's inference, see Evidence C-8).
The entry for Okudaira Sadaharu in the Kansei choshu shokafu (Kokumin Tosho edition, part 3, book 546) states that, since Hideaki had long been in secret contact, Okudaira went to the camp on Mount Matsuo on Ieyasu's orders on the 14th to see how things stood, and stayed on in the camp with Ban Gohei Shigemori. The entry for Kuroda Nagamasa states that "Hideaki had been on Mount Matsuo from the start." Both are Edo-period house genealogies. In Chapter 3 of this book, Ieyasu sends no envoy to Mount Matsuo (Evidence C-3).
In a letter from Ishida Mitsunari said to be addressed to Mashita Nagamori and dated the 12th day of the 9th month, Mitsunari approves as "quite reasonable" Mashita Nagamori's proposal to put the Chugoku forces into the castle at Matsuo and elsewhere; neither "Kingo" nor "Chikuzen Chunagon" is named. No original of this letter has been confirmed, however, and because it survives as a later copy in the Kokon shosokushu (Army General Staff, ed., Nihon senshi: Sekigahara eki, monjo [Japanese Military History: The Sekigahara Campaign, Documents], no. 104, pp. 74–78), Shiramine Jun and others have pointed out that it may be a forgery. The same letter says that among the allies with large armies who came out by the Sawayama road there is someone in contact with the enemy, but gives no name. "Chugoku forces" is a term for the Mori side's forces from the Chugoku region: Mori Hidemoto and Kikkawa Hiroie on Mount Nangu are among them, not a separate body of troops. This book does not use this letter as part of the framework of Mitsunari's original plan. It is listed here only as a source that may be a forgery.
The story of the probing shots (toi-deppo), in which Ieyasu had guns fired at Mount Matsuo to push Hideaki to a decision, is not found in contemporary sources. It is a story that appears in Edo-period house genealogies and war chronicles (an early example is Ii-ke Keichoki [Ii House Record of the Keicho Era], in a copy made in Kanbun 12, 1672), and Shiramine Jun, "Fikushon to shite no 'toi-deppo'" [The "Probing Shots" as Fiction] (parts 1 and 2, 2013), holds that it is a later invention. Kasaya Kazuhiko, Ronso Sekigahara kassen [The Battle of Sekigahara in Dispute] (Shincho Sensho, 2022), on the other hand, regards the volley at the summit as a later embellishment, but argues that there was firing disguised as a mistake, as the Bizen rojin monogatari relates. His evidence (the Bizen rojin monogatari, the Kuroda kafu and the Toda Samon oboegaki) also consists of Edo-period compilations and memoranda. The conventional picture, that Hideaki "did not move until midday, and then, after the probing shots, suddenly switched sides," has been challenged by the research cited in Evidence C-5, in the item on the timing of the main battle. This book does not use this story.

Evidence C-5The Move from Ogaki to Sekigahara

That Mitsunari was at Ogaki from the 8th month and faced the Eastern Army at Akasaka, that the main Western Army left Ogaki on the night of the 14th day of the 9th month of Keicho 5 (1600), and that the Western Army was defeated in the fighting on the 15th can be confirmed from contemporary letters. What happened at Ogaki on the night of the 14th, why the army left the castle, and the deployment and the timing of the fighting on the 15th rest on memoranda written in later years and on researchers' reconstructions, and views differ.

Ishida Mitsunari was already in Ogaki Castle by the 10th day of the 8th month. This comes from a letter from Ishida Mitsunari to Sanada Masayuki and Nobushige of the same date (Aichi kenshi [History of Aichi Prefecture], shiryo-hen [source volumes] 13, Shokuho [Oda–Toyotomi period] 3, no. 938). On the 27th day of the 8th month, Mitsunari, Konishi Yukinaga, Shimazu Yoshihiro and Ukita Hideie jointly issued a prohibition notice (kinzei) to Hayashi village (today Hayashi-cho, Ogaki City), and the four are thought to have been at Ogaki at that point (Gifu kenshi, shiryo-hen, Kodai/Chusei 1, p. 562).
Gifu Castle fell on the 23rd day of the 8th month. A letter from Tokugawa Ieyasu to Asano Nagamasa dated the 28th day of the 8th month says that he will set out, since word has come that the castle was taken on the 23rd (Aichi kenshi, shiryo-hen 13, Shokuho 3, no. 1001).
After Gifu Castle fell, the Eastern Army advanced as far as Akasaka and faced Ogaki. This comes from a letter from Mashita Nagamori to Kikkawa Hiroie dated the 26th day of the 8th month (Dai Nihon komonjo: Kikkawa-ke monjo [Kikkawa family documents], vol. 1, no. 725) and a letter from Yamauchi Kazutoyo dated the 27th day of the 8th month (Yamauchi-ke shiryo: Daiichidai Kazutoyo-ko ki [Yamauchi Family Historical Materials: Record of Lord Kazutoyo, the First Generation], p. 371). The latter writes that, since Ukita, Konishi and Mitsunari are holed up in Ogaki Castle, siege forts (tsukejiro) have been built to surround it.
According to the copy of the jointly signed letter from Ishikawa Yasumichi and Hikosaka Motomasa dated the 17th day of the 9th month (Evidence C-1), the main Western Army left Ogaki around the fifth hour of the night on the 14th, that is, around 8 p.m. A garrison is said to have been left in Ogaki Castle (the Army General Staff's Nihon senshi: Sekigahara eki). One account puts the Western Army's departure from Ogaki at around 7 p.m. on the 14th and the Eastern Army's first movement at around midnight, as the 15th began (Owada Tetsuo, "Sekigahara kenkyu saizensen 2025" [The Sekigahara Research Front Line 2025], in Gifu Sekigahara Battlefield Memorial Museum, ed. and pub., Sekigahara Kenkyukai kikoshu 2025 [Contributions to the Sekigahara Study Group, 2025], pp. 4–5). These times, however, are reconstructions made by comparing the sources, not absolute times. This book sets the Western Army's departure at the fifth hour of the night given in the copy of the jointly signed letter, and follows the account in which the Eastern Army entered the basin after the date had changed.
That on the night of the 14th day of the 9th month word reached the Western Army at Ogaki that Kobayakawa had gone over to Ieyasu's side is reported by Miyanohara Saibei's memorandum, Inoue Shuzen's memorandum, and Mizuno Katsunari's later account (Evidence C-1). Miyanohara Saibei's memorandum (oboegaki) (colophon dated Kanbun 3; Kagoshima-ken shiryo: Kyuki zatsuroku shui, Kirokusho shiryo 1, p. 68) says that it "became known at Ogaki on the 14th day of the 9th month," and Inoue Shuzen's memorandum (colophon dated Meireki 3; Kagoshima-ken shiryo: Kyuki zatsuroku kohen 3, no. 1407) says that the report arrived in the middle of a council. Several later records relate that on the 14th day of the 9th month the Western side, suspicious of Hideaki, tried to make contact with him or take him into custody. Inoue Shuzen's memorandum records, as something heard later, that the Western side tried to summon Hideaki into Ogaki and take him hostage, but that he did not come, pleading "indisposition" (fuki), that is, illness. The Toda Samon oboegaki [Memorandum of Toda Samon] (transcription in Chiikishi kenkyu 116, p. 172) has Otani Yoshitsugu go to Hideaki's camp himself to bring him back, only to be refused a meeting because of illness (Kasaya Kazuhiko also cites this), and the entry for Inaba Masanari in the Kansei choshu shokafu (Kokumin Tosho edition, part 4, book 608) says that earlier, at Takamiya, Otani invited Hideaki and sent Hiratsuka Tamehiro and Toda Shigemasa to ask for a meeting, but Hideaki pleaded illness and did not see him. The methods do not agree, and there is no confirmation from contemporary sources. This book adopts only what these accounts have in common, and in the text presents it as a summons from Mitsunari's side in response to the report that Hideaki had entered Mount Matsuo.
Views differ on why the Western Army left Ogaki. Besides the theory that it was responding to a report that the Eastern Army would pass Ogaki by and advance westward, research that gives weight to accounts in the line of Keicho nenchu Bokusai ki has argued that it may have been a move in response to rumors of treachery by Kobayakawa. On the report of the bypass, a memorandum by another Shimazu retainer, Kanbe Gohee (reading of the surname uncertain), with a colophon dated Kanbun 4 (Kagoshima-ken shiryo: Kyuki zatsuroku kohen 3, no. 1403), says that a council decided to go out to Sekigahara and fight when word reached Ogaki that Ieyasu would leave a holding force at Ogaki and pass on to Kyoto. The same Shimazu memoranda contain both the rumor and the report of the bypass, and the two do not exclude each other. On the rumor, the middle volume of Keicho nenchu Bokusai ki (Shiseki shuran 26) says that Ukita, Konishi and Ishida left Ogaki to deal with the situation because there was a rumor that the Chikuzen Chunagon was plotting treachery. Owada Tetsuo's article (item above) holds that Mitsunari at first had in mind a decisive battle pivoting on Ogaki Castle, in which a relief force, the large armies of Mori Hidemoto, Kikkawa Hiroie and Kobayakawa Hideaki, would strike the approaching Eastern Army from behind; and it judges it highly likely that, uneasy at Kobayakawa's disquieting movements and at the movements of the Mori and Kikkawa on Mount Nangu, Mitsunari moved to Sekigahara on his own judgment. The same article evaluates this situation as the result of Ieyasu's efforts to win over Kobayakawa and to keep Kikkawa out of the fighting. This book treats the rumor of Kobayakawa's treachery (the occupation of Mount Matsuo) as having also contributed to the decision to leave the castle (how much cannot be determined; Analysis II-4), and presents Mitsunari, on hearing that Kobayakawa had entered Mount Matsuo, as leaving Ogaki on his own judgment and moving to Sekigahara. What this book does not adopt is the line that he left the castle solely in response to the Eastern Army's advance; that Mitsunari's original plan counted on Kobayakawa is written just so in Chapter 4. Mizuno Tomoki, Sekigahara kassen o fukugen suru (2023), on the other hand, devotes a chapter to a critical examination of the view that the Western Army moved to Sekigahara to deal with Kobayakawa's change of sides. The point is not settled.
The letter from Tokugawa Ieyasu to Date Masamune dated the 15th day of the 9th month of Keicho 5 says, "Today, the fifteenth, at the hour of the Horse [around noon], we joined battle at Yamanaka in Noshu [Mino Province]," and goes on to say that he defeated the forces of Ukita, Shimazu, Konishi and Ishida and headed for Sawayama. On the day of the battle, Ieyasu himself wrote of the battlefield as "Yamanaka in Noshu." The copy of the jointly signed letter of the 17th day of the 9th month says, in substance, that Ieyasu took the field at Sekigahara at the hour of the Snake (around 10 a.m.) on the 15th and joined battle. The account in Shiramine Jun, "Keicho 5-nen 6-gatsu – donen 9-gatsu ni okeru Tokugawa Ieyasu no gunji kodo ni tsuite" (part 2), reads this hour of the Snake as referring to Ieyasu moving his headquarters forward from the command post on Mount Momokubari (Momokubariyama), to which he had moved from Akasaka, to the front. That on the morning of the 15th Ieyasu advanced from Akasaka to Mount Momokubari and set up his command post there follows the same study (part 2, p. 56). The start of the main battle is often said to have been around 8 a.m. on the 15th day of the 9th month, and it is difficult to fix the exact times of its start and end from the sources. Some studies see about two hours as having passed before the outcome was decided. The copy of the jointly signed letter of the 17th day of the 9th month records that fighting began at the hour of the Snake, and that as the armies engaged ("torimusubi soro koku"), Kobayakawa, Wakisaka, and Ogawa and his son went over to Ieyasu's side and the enemy was routed. On this basis, there is research holding that Kobayakawa switched sides at about the start of the battle and that the Western Army collapsed within a very short time (Shiramine Jun). This research does not accept the picture of the battle as a long struggle lasting more than half a day. Views also differ on how to divide the battlefield and the stages of the fighting. This book is written to follow this research, which reads the copy of the jointly signed letter as Kobayakawa turning at about the start of the battle. The sequence in which Hideaki first sent his vanguard against Otani as the fighting began, and committed his whole army only after confirming that the four units had switched sides, is the author's inference (Evidence C-8). Against this, Mizuno Tomoki rejects the view that Kobayakawa changed sides as the battle began and that the battle ended in a short time ("Sekigahara kassen ni kansuru shinsetsu no kento," Juroku seikishi ronso 18, 2023; Sekigahara kassen o fukugen suru also gives it a chapter). A view that Otani was destroyed by Kobayakawa before the main fighting began, and that the main forces clashed only after that (a two-stage battle), has also been discussed. On those views, this book's reading that Kobayakawa attacked Otani early could stand, but the time order in which he turned almost as the battle began and the Western Army collapsed in a short time would change. This point too is not settled.
That the fog lay thick on the morning of the 15th and was slow to clear comes from the middle volume of Keicho nenchu Bokusai ki, the Todaiki [Record of the Present Age] (compiled in the early Edo period; in Kokusho Kankokai, ed., Shiseki zassan 2, 1911) and the Army General Staff's Nihon senshi: Sekigahara eki, all of them later records.

Evidence C-6Otani and the Four Units

That Otani was killed in battle, and that Kobayakawa, Wakisaka and Ogawa went over to Ieyasu's side, can be confirmed from a letter written two days after the battle. When and where Otani set up camp, what the relationship between the four units and Otani was, and what happened before the four units switched sides rest largely on accounts in research and on later records.

The copy of the jointly signed letter from Ishikawa Yasumichi and Hikosaka Motomasa dated the 17th day of the 9th month of Keicho 5 (1600) (Evidence C-1) states, in substance, that the Western Army was quickly defeated because Kobayakawa Hideaki, Wakisaka Yasuharu, and Ogawa Suketada and his son went over to Ieyasu's side, and it records the name of Otani Gyobu-shoyu (Otani Yoshitsugu's court title) among the commander-rank men killed in battle. In the main battle on the 15th day of the 9th month (October 21, 1600), Kobayakawa Hideaki went over to Ieyasu's side and made the move that led directly to the Western Army's defeat. Where he was camped on that day is covered in Evidence C-4. Of the four units at the foot of Mount Matsuo, the ones this copy of the jointly signed letter names go only as far as Wakisaka and Ogawa and his son; the names of Akaza Naoyasu and Kutsuki Mototsuna do not appear. What these two did relies heavily on later records, and the corroboration is weak. Shiramine Jun, in Shinkaishaku Sekigahara kassen no shinjitsu [The Truth of the Battle of Sekigahara: A New Interpretation] (2014), has raised the possibility that these two did not take part at Sekigahara at all. It may be that the four units did not move as a single body from the start, but were a mix of those who had already come to terms with the Eastern Army and those who went along with the tide on the spot (this book's inference; which house was which cannot be determined from the sources). This book writes the four units as being present (see the item on the positions of the four units below).
In the summer of that year, Otani Yoshitsugu was in charge of the northern front (Hokkokuguchi). He is said to have worked to win over the commanders in the Echizen and Kaga area and to have held Maeda Toshinaga in check from moving south. The Army General Staff's Nihon senshi: Sekigahara eki states that Otani, on receiving an urgent message from Mitsunari, left Echizen and entered Mino on the 3rd day of the 9th month. The “northern front” that Otani had charge of over the summer is, as Ishida Mitsunari's “list of troop strengths for the defenses” (sonae no ninzugaki) dated the 5th day of the 8th month puts it—“commander of the 30,100 troops of the northern front”—the name of a sector of the front in the direction of the Hokuriku region. It is not the name of a road at Sekigahara. The list is a contemporary document, but its breakdown—that is, which daimyo were placed under whom—has not been confirmed.
The account that Otani took up position at the beginning of the 9th month on the Fujikawa plateau (Fujikawadai), at the western end of Sekigahara, comes from Tonooka Shin'ichiro, Otani Yoshitsugu (Ebisu Kosho Shuppan, 2016), p. 78. The phrasing “early in the 9th month, on the Fujikawa plateau” widely seen today comes almost entirely from here.
Separately, there is a tradition that he had set up camp at Yamanaka from the 3rd day of the 9th month. However, this date of the 3rd is not found in contemporary sources; it goes back only to the Army General Staff's Nihon senshi: Sekigahara eki (item above), which cites no source, and Tonooka's account too goes only as far as “early in the 9th month” and does not pin down a date. It is an explanation that includes tradition.
There is also an explanation that Otani Yoshitsugu redeployed from the camp he had built in the village of Yamanaka at the beginning of the 9th month to the Fujikawa plateau (Kuchikitano), to the east. This is a later account based on Sekigahara kassen shi [Record of the Battle of Sekigahara] and other works. The place signposted today as the site of Otani Yoshitsugu's camp is on the side of the earlier camp in the village of Yamanaka. As for when he moved, Sekigahara kassen zushi puts it on the morning of the 15th, while Sekigahara gunki taisei already treats the Fujikawa plateau as Otani's camp on the night of the 14th. This book adopts this explanation of the change of position, and writes it as Otani moving his camp closer, on Mitsunari's instructions, on the night of the 14th day of the 9th month, when Mitsunari decided to move to Sekigahara. That instruction is the author's inference (Evidence C-8).
A letter from Ikeda Terumasa to Honda Masazumi dated the 7th day of the 10th month states that Otani arrived in the Sekigahara area on the 14th day of the 9th month. It is a document from about three weeks after the battle, and it does not agree with the account above that Otani was in the Yamanaka area from the beginning of the 9th month. Takahashi Yosuke's view that the one at Ogaki on the 14th was Otani is in Evidence C-4. This book adopts the account that Otani was in the Yamanaka area from the beginning of the 9th month.
That Yamanaka was part of the battlefield on the day is confirmed by Ieyasu's own letter (Evidence C-5). That is separate from the 3rd day of the 9th month resting only on tradition.
Shiramine Jun, drawing on records handed down in the Todo Shinshichiro house, has proposed the theory that Otani was deployed below Mount Nangu (“Otani Yoshitsugu no Nangusan-ka fujin setsu ni tsuite” [On the Theory That Otani Yoshitsugu Was Deployed Below Mount Nangu], Kokushi sanshu 27, 2025). The “Shinshichiro kajo” [House Records of the Shinshichiro Line], the “Shinshichiro kerai Hama Ichiemon oboegaki” [Memorandum of Hama Ichiemon, Retainer of the Shinshichiro Line], and the “Todo-sei shoke to kafushu: Todo Shinshichiro-ke” [Collected Genealogies of the Houses Bearing the Todo Name: The Todo Shinshichiro House] contain passages indicating that Otani's side and Todo's side fought on or below Mount Nangu. This does provide material for reconsidering the conventional deployment. Whether it shows where Otani Yoshitsugu himself was, however, or fighting with part of Otani's force, still needs examination. This is because in war chronicles and house records, a phrase such as “attacked so-and-so” sometimes refers not to the commander himself but to his sector or his troops.
This book does not adopt the below-Mount-Nangu theory; it writes the received deployment (the Fujikawa plateau).
The account that it was Todo Takatora and Kyogoku Takatomo who fought facing Otani's unit rests on records from the Todo house side, such as Todo-ke oboegaki [Memoranda of the Todo House] and Kozan-ko jitsuroku [True Record of Lord Kozan], and on the Army General Staff's Nihon senshi: Sekigahara eki; it cannot be confirmed as far as contemporary sources. Shiramine Jun's reconstruction has changed with the year of the study. In a 2016 study, he reconstructed from a group of memoranda by Shimazu retainers that Otani's unit jutted out east of the Western Army's main body and came under concentrated attack, and the names Todo and Kyogoku do not appear there. In a 2018 study, he supported the Todo–Kyogoku account on the basis of the same Todo house sources, and in 2025 he put forward the below-Mount-Nangu deployment theory above. This book adopts the Todo–Kyogoku account and writes Todo and Kyogoku as engaging Otani from the basin side.
The received view of the deployment is that the four units of Wakisaka Yasuharu, Kutsuki Mototsuna, Ogawa Suketada and Akaza Naoyasu were in the same area by about the time Otani entered Yamanaka. This book adopts it and writes the four units as present throughout. This book's view that the four units came to stand between Otani and Mount Matsuo only after Kobayakawa entered the mountain is in Evidence C-8.
The account that calls the four units “Otani Yoshitsugu's yoriki (attached commanders)” goes back to Tonooka Shin'ichiro, Otani Yoshitsugu, p. 78. The descriptions widely seen today come almost entirely from here. The way of counting in the same passage, which adds Otani's force and the four yoriki units together, is in Evidence A-6. The Army General Staff's Nihon senshi: Sekigahara eki simply lists the four units, each as an independent commander, in the order of their positions, and does not use the word yoriki. In Akaza's case, it can be confirmed from a jointly signed letter dated the 1st day of the 10th month of Keicho 4 (1599) that he was a yoriki of Horio Yoshiharu. No contemporary document is found that shows them attached to Otani as yoriki. In the narration, this book writes them as four independent houses in the same sector as Otani and does not call them yoriki; in the dialogue, the men of Akaza's camp call themselves Otani's yoriki (Chapter 5). It cannot be said that they were attached to him as yoriki as an institution, but neither is the relationship itself denied.
The Wakisaka house was in contact with Ieyasu's side before the battle. The evidence is a letter from Tokugawa Ieyasu dated the 1st day of the 8th month of Keicho 5 (1600) (in the Wakisaka-ke monjo [Wakisaka family documents] and elsewhere), and the go-between in it is Yamaoka Doami. It is not Todo Takatora. Opinions differ on whether it was addressed to Yasuharu or to his second son, Yasumoto; in the transcription that gives Yasumoto as the addressee, Ieyasu asks father and son to consult together. It is natural to see Yasuharu as having shared in this contact, but it cannot be said that Yasuharu himself received the letter. The route by which Wakisaka and Ogawa had been in secret contact with the Eastern Army through Todo Takatora, on the other hand, is set out by Fujita Tatsuo in Todo Takatora ron [On Todo Takatora] (Hanawa Shobo, 2018), citing the “Hikakushu” [Collected Secret Memoranda], and the Edo-period Kozan-ko jitsuroku and Keicho gunki [Military Chronicle of the Keicho Era] say much the same. It is memoranda on the Todo house's side that record the prior arrangements between Todo and Wakisaka and Ogawa in concrete terms; there are reservations about whether their content is true, and when and what was promised has not been established. That Ogawa had been in secret contact with the Eastern Army beforehand appears only in these house-record accounts. Wakisaka's prior contact can be confirmed from a contemporary document, but the route through Todo has not been confirmed, and for Ogawa the secret contact itself rests on later records. This book does not touch on this route in the narrative; it takes it up in Analysis I-8 and Analysis II-3, and lists it in Evidence A-5 in the rows for Wakisaka and Ogawa and in the row for Todo (the later Todo house records). For Kutsuki, too, the entry for Kutsuki Mototsuna in the Kansei choshu shokafu (book 415) states that at Sekigahara Mototsuna joined Otani's command, was in secret contact through Todo Takatora together with Wakisaka and Ogawa, and attacked Otani's camp together with Hideaki. This is an Edo-period house genealogy and cannot be confirmed from contemporary sources. For Akaza, there is no record of secret contact.
The entry for Kobayakawa Hideaki in the Kansei choshu shokafu (Kokumin Tosho edition, part 4, book 618) states that on the 14th Wakisaka Yasuharu, Kutsuki Mototsuna and Ogawa "were of one mind with Hideaki and camped together with him," and that after the battle, when Hideaki went to Ieyasu's camp with Murakoshi Naoyoshi as his guide, the three houses followed him. It is an Edo-period house genealogy, and it does not name Akaza. It cannot be confirmed from contemporary sources.
The depiction of Otani Yoshitsugu keeping his own 600 men by him, and of those 600 pushing back the Kobayakawa forces coming down the mountain two or three times, derives from war chronicles such as Sekigahara gunki taisei, and is not corroborated by contemporary sources. That Otani's force put up some resistance to Kobayakawa's attack is itself possible, but neither the number of times it pushed them back nor the numbers of men can be confirmed. This book adopts this depiction in Chapter 6. Details such as the order of guns and spears and the shoving at the edge of the plateau are invented (Evidence C-9).
Otani Yoshitsugu's unit is said to have been attacked from several directions and destroyed. Who attacked from which direction comes from the narrative of the Army General Staff's Nihon senshi: Sekigahara eki and is not written in contemporary documents. The Western Army is said to have fled one unit after another once Kobayakawa switched sides. The link between Kobayakawa's switching sides and the defeat of Otani's unit is written in a contemporary diary kept in Kyoto: the entry for the 15th day of the 9th month in Shunkyuki [Diary of Bonshun] records Otani Gyobu-shoyu's death in battle and attributes it, in the next breath, to the betrayal of Kobayakawa Hideaki. Bonshun, in Kyoto, wrote down what he had heard; it is not the record of anyone on the battlefield.
No source survives that shows what was actually conveyed to the four units, who served as envoy, or which house made up its mind in what order. The way this book depicts it is supplied as invention (for the invented elements of the scene on the night before in the Story, see Evidence C-9).
Those who switched sides were not treated uniformly after the war. Wakisaka was confirmed in his original domain, and Kutsuki too was confirmed in his original domain (the Kansei choshu shokafu gives it as over 9,590 koku). Akaza was stripped of his domain. So was Ogawa. Ogawa too is said to have been in secret contact with the Eastern Army beforehand (item above). In Ogawa's case, later records such as Kaisei Mikawa go fudoki [Revised Later Gazetteer of Mikawa] report that his relationship with Mitsunari and his everyday reputation were called into question.

Evidence C-7Mount Nangu and Kikkawa

That Kikkawa Hiroie had been in secret contact with the Eastern Army can be confirmed from contemporary written oaths (kishomon). The scene in which Kikkawa blocked the foot of the mountain on the 15th and kept the Mori forces from coming down rests on records put in order afterward by a party with an interest in them.

That Kikkawa Hiroie had been in secret contact with the Eastern Army can be confirmed from the jointly signed written oath of Honda Tadakatsu and Ii Naomasa dated the 14th day of the 9th month of Keicho 5 (1600), and from the jointly signed written oath of Fukushima Masanori and Kuroda Nagamasa of the same date (both in the Mori-ke monjo [Mori family documents]). His arrival at Mount Nangu is based on a letter from Kikkawa Hiroie to Soshiki Nagayoshi dated the 12th day of the 9th month.
The forces on Mount Nangu from the 14th to the 15th day of the 9th month are said to have been those of Mori Hidemoto, Kikkawa Hiroie, Chosokabe Morichika and Ankokuji Ekei (the deployment maps of the Army General Staff's Nihon senshi: Sekigahara eki). Kikkawa's arrival is confirmed by the letter in the item above, but the presence of the other three cannot be confirmed from contemporary documents. That these forces did not come down the mountain on the 15th and emerge behind the Eastern Army is the narrative of the Army General Staff's Nihon senshi: Sekigahara eki; no contemporary document has been found that states it directly.
The scene in which Kikkawa Hiroie took up position at the foot of Mount Nangu and Mori Hidemoto and the others could not come down rests on records from the Kikkawa house side and on Edo-period war chronicles. Kikkawa's secret contact with the Eastern Army itself can be confirmed from the written oaths (item above), but the scene of his holding Mori Hidemoto back is another matter. Yamamoto Hiroshi, in “Intoku taiheiki no seiritsu jijo to Kikkawa-ke no kakaku senden katsudo” [The Making of Intoku taiheiki and the Kikkawa House's Efforts to Promote Its Standing] (Yamaguchi-ken chihoshi kenkyu 93, 2005), argues that the Kikkawa house's Iwakuni domain worked to have passages justifying Kikkawa Hiroie included in Intoku taiheiki and Sekigahara gunki taisei, and Mitsunari Junji, noting that the group of letters included in the “Kikkawa kafu” [Kikkawa House Genealogy] contains words that were not in use at the time, has pointed out that they may be forgeries. It includes the possibility of being a story put in order afterward by a party with an interest in it.

Evidence C-8The Author's Own Inferences, and the Assumptions Made for the Analysis

Evidence C-8 lists the interpretations in this book that are not written in the sources as they stand, divided into the author's own inferences and the assumptions made for the analysis. Most of the sources its items touch on are in the sections for Evidence C-2 to C-7, so this section gives the sources that appear only in Evidence C-8, and where to look for each item.

The item that what Hideaki had decided in advance went no further than a broad course (side with the Eastern Army, do not announce it early, commit no troops elsewhere until their role is clear), and that the conditions for beginning to act (begin once Ieyasu has left Kiyosu, and enter Mount Matsuo) were set by Hideaki himself. The Story depicts him communicating these conditions to Ieyasu's side and thereby reaching an arrangement with them; Evidence C-8 lists that arrangement as an assumption made for the analysis, and the analysis does not presuppose it. Entering Mount Matsuo only after hearing that Ieyasu had entered Mino is something Hideaki decided after the news that Ieyasu had left Kiyosu, and it was not communicated to Ieyasu's side (Chapter 3, Analysis I-10). No surviving document records such an arrangement.
The material for dividing the possible extent of the arrangement into four levels (the Eastern Army made an approach; Hideaki's side responded in some form; they had agreed on his siding with the Eastern Army; they had arranged the place and the time to begin action) is the jointly signed letter of Kuroda Nagamasa and Asano Yoshinaga of the 28th day of the 8th month; the Keicho nenchu Bokusai ki, which has envoys from Hideaki coming on the 3rd and 8th days of the 9th month; the entry for Kuroda Nagamasa in the Kansei choshu shokafu, which has hostages exchanged with Kuroda Nagamasa; the written oath of Honda Tadakatsu and Ii Naomasa to Hiraoka and Inaba of the 14th day of the 9th month (a copy in Sekigahara gunki taisei); and Ieyasu's letter of the 24th day of the 9th month (a copy in "Komonjoshu"). Where each is published, and its wording, are given in Evidence C-3. The letter of the 28th day of the 8th month confirms only the first level; the other four suggest the second level or more, but do not decide between the third and the fourth.
Takahashi Yosuke, Shin Sekigahara (Kodansha Gendai Shinsho, 2025), offers an interpretation in which Hideaki was acting as part of Ieyasu's side even before the battle. His evaluation of the sources he uses and his method of argument have been criticized by Goza Yuichi, Mizuno Tomoki and others (2026). This book does not adopt this interpretation.
The assumption that Mitsunari, when he decided on the night of the 14th day of the 9th month to leave Ogaki, already knew that Ieyasu had arrived at Akasaka. Ieyasu's arrival at Akasaka on the 14th can be confirmed from letters (Evidence C-4), but no contemporary source records when Mitsunari learned of it. The middle volume of Keicho nenchu Bokusai ki (Shiseki shuran 26) says that men were sent out from Ogaki three times to confirm Ieyasu's arrival before the Western Army left the castle, but it is a later record by Itasaka Bokusai, who accompanied Ieyasu.
The inference that Hideaki took Ieyasu's departure from Kiyosu in Owari as his cue to leave Takamiya, and Ieyasu's own entry into Gifu as his cue to enter Mount Matsuo. That Ieyasu was at Gifu on the 13th day of the 9th month and at Akasaka on the 14th can be confirmed from letters (Evidence C-4), but no source describes a causal link between Ieyasu's itinerary and Hideaki's movements. Dating Ieyasu's departure from Kiyosu to dawn on the 12th is also the author's inference: the middle volume of Keicho nenchu Bokusai ki (the Gajikan gasho edition, leaf 9 and elsewhere) has him arrive at Kiyosu on the 11th, stay there through the 12th, and set out from Kiyosu for Gifu on the 13th, and the Tanaka kohaiki (1822) says the same.
The assumption that before dawn on the 15th the four houses at the foot of the mountain were told that the Kobayakawa forces would advance toward Otani, and that each house should decide for itself whether to follow them toward Otani or stay on Otani's side. There is no source showing that anything was conveyed to the four houses, and the envoys and their spoken messages are fiction (Evidence C-9; see also the item in Evidence C-6 on what was conveyed to the four units). Analysis II-3 does not treat this message as required for the model of the four houses' opening moves.
The sources touched on by the other items are in the sections on the siege of Fushimi and Ieyasu's coordination by letter (Evidence C-2); the letter of the 28th day of the 8th month and whether it was answered in writing (Evidence C-3); the entry onto Mount Matsuo and the view that by the opening of the battle he had moved to the rise in front of it (Evidence C-4); the records of the Western side trying to summon Hideaki to Ogaki, and the time of the main battle (Evidence C-5); the list of troop strengths of the 5th day of the 8th month, Akaza's attachment as a yoriki, and Otani's change of position (Evidence C-6); the forces on Mount Nangu (Evidence C-7); and the commissioners' authority (the item on the Five Elders and Five Commissioners under Evidence C-1 above).

About the bookBetrayal as Strategy: overview and excerpt