Which 1600 battle did Kobayakawa Hideaki’s defection help decide in Tokugawa Ieyasu’s favor?

The Battle of Sekigahara was decided in Tokugawa Ieyasu’s favor after Kobayakawa Hideaki defected from the western coalition.

Fought on 21 October 1600, Sekigahara brought together armies supporting Ieyasu and forces loyal to the Toyotomi regime under Ishida Mitsunari. Hideaki commanded troops positioned on Mount Matsuo and initially remained uncertain about entering the battle.

Ieyasu ordered gunfire toward Hideaki’s position, after which Hideaki attacked the western army. Other units followed, and the shift helped collapse the coalition’s position. The battle ended in a decisive victory for Ieyasu and made him the uncontested military power in Japan.

The victory led to Ieyasu’s appointment as shogun in 1603 and the establishment of the Tokugawa shogunate. Sekigahara is sometimes described as a simple east-versus-west struggle, but many daimyo acted from complicated combinations of loyalty, calculation, and local interest.