Sagara Sei's 'Betrayal as Strategy' Reexamines Sekigahara Betrayal

Sagara Sei has released a provocative new book, Betrayal as Strategy: Kobayakawa Hideaki and the Game Theory of Sekigahara, now available on Amazon Kindle in both English and Japanese. The paperback edition is set to follow in mid-October 2026. The book tackles one of Japanese history's most infamous acts of battlefield treachery: the decision of eighteen-year-old Kobayakawa Hideaki to switch sides during the 1600 Battle of Sekigahara, effectively handing victory to Tokugawa Ieyasu. Rather than accepting the traditional narrative of a hesitant youth coerced by Ieyasu's 'probing shots'—an episode absent from contemporary sources—Sagara proposes a bold hypothesis: Hideaki was not indecisive but a strategic genius who independently chose his moment to defect. Assuming no one ordered Hideaki to occupy Mount Matsuo with his 8,000 men, the book reconstructs the battle as a strategic game, analyzing the forces, terrain, and decisions through game theory. It argues that Hideaki's actions—concealing his allegiance, seizing the mountain, and attacking Otani Yoshitsugu—formed a coherent line of decisions aimed at preserving his house. The book claims no new sources; instead, it carefully separates evidence from inference, presenting the story as a novel while explaining the reasoning in an Analysis section and marking the boundaries of fact and fiction in an Evidence section. For those interested, the book page is available at https://sagara-sei.com/en/books/sekigahara/ and the Kindle edition can be purchased at https://www.amazon.com/dp/B0HLDHRPKC for US$5.99. A press kit with cover and map images, factsheet, and media sample is also available at https://sagara-sei.com/en/books/sekigahara/press/.