A historical novel, published in English and Japanese on October 2, 2026, reconstructs the decisions of the eighteen-year-old lord whose change of sides decided the battle, using the sources and game theory.
TOKYO, JAPAN, October 02, 2026 /24-7PressRelease/ -- Sagara Sei publishes Betrayal as Strategy: Kobayakawa Hideaki and the Game Theory of Sekigahara on Amazon Kindle on October 2, 2026, in English and in the original Japanese (裏切りの戦略). A paperback edition follows (Coming mid-October 2026).
Sekigahara, and the question historians have asked
In 1600 the Battle of Sekigahara decided who would rule Japan, and one commander's change of sides decided the battle. Kobayakawa Hideaki, eighteen years old, held Mount Matsuo (Matsuoyama) 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 hypothesis: why assume anyone put him there?
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.
What the hypothesis shows
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.
Evidence and inference, kept apart
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.
Publication details
Title: Betrayal as Strategy: Kobayakawa Hideaki and the Game Theory of Sekigahara
Japanese edition: 裏切りの戦略──再考・小早川秀秋
Author: Sagara Sei (相良 静)
Kindle edition: Available now, US$5.99
Paperback: Coming mid-October 2026, Expected price US$12.99
Book page: https://sagara-sei.com/en/books/sekigahara/
Amazon (Kindle edition): https://www.amazon.com/dp/B0HLDHRPKC
Press kit (cover and map images, factsheet, media sample): https://sagara-sei.com/en/books/sekigahara/press/
Sagara Sei rereads historical decisions through strategy, institutions and the structure of games.
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