What happened
The largest field battle of the samurai age — total forces are commonly put at some 150,000 or more — was fought at Sekigahara in Mino (modern Gifu). Tokugawa Ieyasu’s Eastern Army faced the Western coalition organized by Ishida Mitsunari in the name of the Toyotomi cause.
The battle was decided within a day, pivotally by the defection of Kobayakawa Hideaki to the Tokugawa side. Mitsunari and other Western leaders were executed, and Ieyasu massively redistributed domains to reward allies and weaken enemies.
Background
Toyotomi Hideyoshi’s death in 1598 left a power vacuum, and the confrontation at Sekigahara grew out of it.
Consequences
Tokugawa hegemony followed: Ieyasu became shogun in 1603, and the Toyotomi house was destroyed at Osaka in 1614–15.