In 1600, the Battle of Sekigahara determined who would rule Japan, and one commander’s change of sides decided the battle. Kobayakawa Hideaki, eighteen years old, held Mount Matsuo above the field with 8,000 men. He has long been remembered as an indecisive traitor who switched 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.
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 in the original Japanese, 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. From that assumption, it 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. 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 matters because it challenges a centuries-old narrative about one of Japan’s most pivotal battles. If Hideaki’s betrayal was a rational strategy rather than a spur-of-the-moment defection, it alters how historians and enthusiasts understand the mechanics of Sekigahara and the role of individual decision-making in shaping history. The book’s approach—applying game theory to a historical event—could inspire new interpretations of other decisive moments. The Kindle edition is available now for US$5.99, with a paperback edition coming mid-October 2026 at an expected price of US$12.99. For more details, visit the book page, and the Kindle edition can be purchased at Amazon. A press kit with cover and map images, a factsheet, and a media sample is also available.
