Sagara Sei's new book, Betrayal as Strategy: Kobayakawa Hideaki and the Game Theory of Sekigahara, was published on Amazon Kindle on October 2, 2026, in English and in the original Japanese. A paperback edition is expected in mid-October 2026. The book offers a fresh interpretation of one of Japan's most famous battles and the controversial decision that decided it.
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 changed 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.
The book 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, the book 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 publication matters because it challenges a long-held historical narrative and introduces a new methodological approach to understanding past decisions. By applying game theory to a pivotal moment in Japanese history, Sagara Sei invites readers to reconsider not only Hideaki's actions but also how historians interpret ambiguous evidence. The book's structured separation of evidence and inference provides a model for balancing rigorous analysis with engaging storytelling, potentially influencing how popular history is written. For readers interested in Japanese history, military strategy, or decision-making, this book offers a compelling reinterpretation that could spark further debate among scholars and enthusiasts alike.
The book is available now on Amazon Kindle for US$5.99. A paperback edition is coming mid-October 2026, expected price US$12.99. For more information, 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/.
