In 1600, the Battle of Sekigahara determined who would rule Japan, and one commander's change of sides decided the battle. Kobayakawa Hideaki, an eighteen-year-old lord, held Mount Matsuo with 8,000 men. He has long been remembered as an indecisive traitor who only turned against the Western Army when Tokugawa Ieyasu's guns fired on his position. However, that episode—the 'probing shots' (toi-deppo)—does not appear in contemporary sources, and modern research has treated it as a later invention. Historians have also debated who placed Hideaki on the mountain: 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 Japanese, steps back from that debate. The book assumes that Hideaki, watching Ieyasu advance west, chose the place and the moment himself. A lord commanding his own army needs no one else's order to move it. 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 moves open to the other side.
Under this model, 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—read as one line of decisions made to keep his house alive. Far from a hesitant youth, Hideaki emerges as a strategic genius who made the right call at each turn. The book does not claim 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.
For readers interested in Japanese history, military strategy, or game theory, this book offers a fresh perspective on a pivotal moment. It challenges the long-held view of Hideaki as a traitor and instead presents him as a rational actor in a high-stakes game. The implications extend beyond academia: understanding how individuals make decisions under uncertainty can inform modern strategic thinking in business, politics, and beyond. The book is available now on Kindle 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 at https://sagara-sei.com/en/books/sekigahara/ or the Amazon Kindle edition at https://www.amazon.com/dp/B0HLDHRPKC. A press kit with cover and map images, factsheet, and media sample is available at https://sagara-sei.com/en/books/sekigahara/press/.
Sagara Sei rereads historical decisions through strategy, institutions, and the structure of games. This book is a must-read for anyone seeking to understand how one decision can alter the course of history.
