A new reading of Sekigahara, Japan 1600: the betrayal that decided the battle, reinterpreted as strategy

Press release

A New Reading of Sekigahara, Japan 1600: The Betrayal That Decided the Battle, Reinterpreted as Strategy

Sagara Sei publishes Betrayal as Strategy: Kobayakawa Hideaki and the Game Theory of Sekigahara on Amazon Kindle on October 2, 2026, in English and in the original Japanese (裏切りの戦略). A paperback edition follows (Coming mid-October 2026).

Sekigahara, and the question historians have asked

In 1600 the Battle of Sekigahara decided who would rule Japan, and one commander's change of sides decided the battle. Kobayakawa Hideaki, eighteen years old, held Mount Matsuo (Matsuoyama) above the field with 8,000 men. He has long been remembered as an indecisive traitor who changed sides only when Tokugawa Ieyasu's guns fired on his position. That episode, the "probing shots" (toi-deppo), does not appear in contemporary sources, and research has treated it as a later invention. Historians have asked who put Hideaki on Mount Matsuo — the Western Army, or Ieyasu's side — and when he began dealing with Ieyasu. No contemporary document ordering him onto the mountain survives from either side.

The hypothesis: why assume anyone put him there?

The book steps back from that question. A lord commanding his own army needs no one else's order to move it. The book assumes that Hideaki, watching Ieyasu advance west, chose the place and the moment himself.

What the hypothesis shows

From that assumption, the book rebuilds the forces, the terrain, the positions of every unit, Ieyasu's progress and the four houses below the mountain. It treats Sekigahara as a strategic game: who decided, what each side knew, which moves were open, and how one move changed the moves open to the other side. Keeping his allegiance from the Western Army until the day of the battle, taking Mount Matsuo, and moving on Otani Yoshitsugu the moment the battle opened then read as one line of decisions, made to keep his house alive. On this model, Hideaki was not a hesitant youth but a strategic genius who made the right call at each turn.

Evidence and inference, kept apart

The book claims no new sources. It keeps every event the sources record, infers the decisions the sources alone cannot settle, using game theory and decision-making under uncertainty, then tells them as a novel. The story follows Hideaki's actions in order; the Analysis explains why he chose each one and tests that reading against other explanations and the sources; the Evidence section marks where contemporary documents end and later records, modern research, received tradition, the author's inference and assumptions, and fiction begin.

Publication details

Title	Betrayal as Strategy: Kobayakawa Hideaki and the Game Theory of Sekigahara
Japanese edition	裏切りの戦略──再考・小早川秀秋
Author	Sagara Sei (相良 静)
Kindle edition	Available for pre-order (releases October 2, 2026), US$5.99
Paperback	Coming mid-October 2026, Expected price US$12.99
Book page	https://sagara-sei.com/en/books/sekigahara/

About the author

Sagara Sei rereads historical decisions through strategy, institutions and the structure of games.

Press contact

Sagara Sei, press@sagara-sei.com

Story angles

• Was Kobayakawa Hideaki really indecisive?
• What changes when Sekigahara is modeled as a strategic game?
• Decision versus action: why a delayed move does not necessarily mean hesitation
• How the ground itself shaped Hideaki's choices — and Ishida Mitsunari's
• How far can historical fiction reconstruct decisions the sources alone cannot settle?

Key points

• Deciding, declaring and moving are treated as separate choices. Hideaki declared for the Eastern Army before the Western Army only on the day of the battle; the book reads this not as indecision but as a choice to keep his side hidden from them
• What the sources cannot settle is marked as open, and a decision model is set on it as a hypothesis. Every assumption the model makes is set out in the Evidence section (Evidence C-8)
• Each player is described by what he knew and what he could choose. Hideaki, Mitsunari, Ieyasu, Otani and the four houses below the mountain, each with its own aims, information and options
• The terrain is checked. The book draws Sekigahara as it could be seen on the morning of the battle from three camps, using elevation data from the Geospatial Information Authority of Japan (the map "The View from Three Camps" at the end of Chapter 5; the terrain is as it is today). From Mount Matsuo, Ieyasu's headquarters at Akasaka lay hidden behind Mount Nangu
• The inference becomes a novel, and the grounds stay in the book. The story reads on its own, but a reader can always check where the sources end and the inference begins

Method

The book does not rewrite history. It infers the decisions left open between the recorded events and tells them as a novel. It works in six steps:

1. Fix the events the sources confirm
2. State which interpretations from existing research it adopts
3. Leave open what the sources cannot settle
4. Set out, for each lord, his aims, what he knew and what he could choose
5. Build a line of decisions that contradicts no source
6. Tell that line of decisions as a novel

It keeps its material in three layers:

Layer	What it is	How the book treats it
Sources	Events confirmed by contemporary documents	Not changed
Research and received views	Historians' interpretations and widely held accounts	Those adopted are named in the Evidence section
Decision model	Judgments the sources alone cannot settle	The author's inference; every assumption is written out in the Evidence section

Contents

The Story　Introduction / 1 The Castle That Would Not Fall / 2 Siding with the East / 3 Taking Mount Matsuo / 4 The Western Army Leaves Ogaki / 5 Turning the Defensive Line / 6 Concentrating Force at One Point / Two Years Later / How This Book Reads the Story

The Analysis　I Kobayakawa Hideaki Through Game Theory / II Testing the Book's Hypothesis

The Evidence　A The Situation, the Forces, and the Positions / B Kobayakawa Hideaki in the Historical Record / C Sources, Tradition, and Fiction

The source behind each point is set out, item by item, in The Sources in Detail.

Downloads

Terms of use　Free to use when reviewing or reporting on the book, with the credit "Betrayal as Strategy, Sagara Sei". Please do not alter the images other than by cropping. The map credits are part of each image; please keep them.

Download everything (ZIP)

The press release, factsheet, media sample, cover and maps below, with a README.txt describing each, in a single file. ZIP

Press release (PDF)

The press release section of this page, set on A4. PDF

Factsheet (one A4 page)

Title, publication dates, prices, summary, the book's hypothesis and contact on a single page. PDF (view on the web)

Media sample

Pages from the paperback typesetting at three points (the opening of Chapter 3, Analysis I-6, and the opening of Evidence C). The Kindle edition has the same text. These are the same pages as the excerpt on the book's page. PDF

Cover

Cover of Betrayal as Strategy: Kobayakawa Hideaki and the Game Theory of Sekigahara. Click for the full-size JPEG (1480 × 2100).

Map 1: The Road to Sekigahara: Movements of the Commanders

The movements of the commanders from the siege of Fushimi Castle to Sekigahara (at the end of Chapter 2).

Map 2: The Eve of Sekigahara: Movements of the Commanders

The movements of the commanders up to the eve of the battle, including the Kobayakawa march from Takamiya by way of Kashiwabara onto Mount Matsuo (at the end of Chapter 4).

Map 3: The Deployment at Sekigahara

The positions on the 15th day of the 9th month of Keicho 5 (October 21, 1600) (at the end of Chapter 5). The positions of the units are estimates based on the received view (Evidence A-9).

Author

Sagara Sei (相良 静) rereads historical decisions through strategy, institutions and the structure of games. Author's page

There is no author photograph.

Contact

For interviews, image permissions and review copies (PDF or EPUB), write to press@sagara-sei.com.