In 1600, the Battle of Sekigahara determined who would rule Japan, and the defection of Kobayakawa Hideaki decided the battle. For over four centuries, Hideaki has been remembered as an indecisive traitor who only switched sides when Tokugawa Ieyasu’s guns fired on his position on Mount Matsuo. That episode, the so-called “probing shots” (toi-deppo), does not appear in contemporary sources and has been treated by modern research as a later invention. Historians have long debated who placed Hideaki on Mount Matsuo—the Western Army or Ieyasu’s side—and when he began negotiating with Ieyasu, but no contemporary document ordering him onto the mountain survives from either side.
Sagara Sei’s new book, Betrayal as Strategy: Kobayakawa Hideaki and the Game Theory of Sekigahara, published on Amazon Kindle on October 2, 2026, in English and in the original Japanese, steps back from that question. A lord commanding his own army, the book argues, 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. From that assumption, it 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 moves available to the other side.
In this reading, Hideaki’s actions—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—form 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. The book claims no new sources. It keeps every event the sources record, infers the decisions that sources alone cannot settle, and uses game theory and decision-making under uncertainty to tell 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; and 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 reinterpretation matters because it challenges a deeply entrenched historical narrative that has often reduced Hideaki to a symbol of betrayal. By applying game theory, the book offers a framework for understanding not only Sekigahara but also other historical decisions where evidence is incomplete. It invites readers to separate evidence from inference and to consider how strategic logic can illuminate actions that appear irrational or treacherous. The book is available in Kindle edition for US$5.99, with a paperback edition expected in mid-October 2026 at approximately US$12.99. For more information, visit the book page or the press kit with cover images, maps, and a media sample.
