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 placed Hideaki on Mount Matsuo—the Western Army or Ieyasu's side—and when he began negotiating 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 question. 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 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 reinterpretation matters because it challenges a centuries-old narrative about one of Japan's most decisive battles. If Hideaki's defection was a calculated strategic move rather than a spur-of-the-moment betrayal, it alters how historians and enthusiasts understand the motivations and decision-making of key figures in the unification of Japan. The book's application of game theory to a historical event also demonstrates how modern analytical tools can shed new light on long-debated questions, potentially inspiring similar approaches in other historical studies. For readers interested in Japanese history, military strategy, or game theory, this work offers a fresh perspective on a familiar story.
The Kindle edition is available now for US$5.99. A paperback edition is expected in mid-October 2026, priced at US$12.99. For more information, visit the book page. The book can be purchased on Amazon. A press kit with cover and map images, factsheet, and media sample is available at https://sagara-sei.com/en/books/sekigahara/press/.
