In 1600, the Battle of Sekigahara determined who would rule Japan. The defection of Kobayakawa Hideaki, an eighteen-year-old commander with 8,000 men on Mount Matsuo, tipped the scales to Tokugawa Ieyasu. History has remembered Hideaki as an indecisive traitor who switched sides only after Ieyasu’s forces fired probing shots at his position. But that episode, known as the “toi-deppo,” does not appear in contemporary sources, and modern research treats it as a later invention. Historians have long debated who placed Hideaki on the mountain and when he began negotiating with Ieyasu, yet no contemporary document ordering him there survives from either side.
A new book by Sagara Sei, Betrayal as Strategy: Kobayakawa Hideaki and the Game Theory of Sekigahara, published on Amazon Kindle on October 2, 2026, in English and Japanese, offers a radical reinterpretation. The book asks why we assume anyone ordered Hideaki onto Mount Matsuo. A lord commanding his own army needs no one else’s order to move it. The book posits that Hideaki, watching Ieyasu advance west, chose the place and moment himself.
From this assumption, the book rebuilds the forces, terrain, 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 other side’s options. Keeping his allegiance from the Western Army until the day of 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. In this model, Hideaki was not a hesitant youth but a strategic genius who made the right call at each turn.
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.
This publication matters because it challenges a centuries-old narrative. If Hideaki was not a traitor but a rational actor, it changes how we understand the Battle of Sekigahara and the nature of betrayal in early modern Japan. The book’s use of game theory to analyze historical decisions offers a new lens for historians and general readers alike. The Kindle edition is available now for US$5.99, with a paperback edition expected in mid-October 2026 at US$12.99. For more details, visit the book page. A press kit with cover and map images, factsheet, and media sample is available at https://sagara-sei.com/en/books/sekigahara/press/. The book is also available on Amazon.
