How did Tokugawa Ieyasu win the Battle of Sekigahara?

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Statue of Tokugawa Ieyasu in ornate robes holding a bow, with arrows on his back, at the Tosho-gu shrine in Nikko, Japan. — Tokugawa Ieyasu Statue of Tokugawa Ieyasu at the Tōshō Shrine in Nikkō, Japan.

Tokugawa Ieyasu secured victory at the Battle of Sekigahara on October 21, 1600, primarily through strategic diplomacy and the subversion of his opponent's coalition.

While Ishida Mitsunari planned to trap Ieyasu’s forces using a pincer movement involving the Kobayakawa and Mōri clans, Ieyasu had secretly secured the loyalty of key western commanders before the fighting began. Specifically:

Kobayakawa Hideaki: Though positioned to flank Ieyasu, Hideaki had secretly promised to fight for the Tokugawa. When Ieyasu forced his hand by ordering arquebusiers to fire on his position, Hideaki attacked the western lines, causing a collapse of the coalition's center.
Kikkawa Hiroie: He communicated with eastern generals to ensure the Mōri clan would remain stationary, effectively neutralizing a massive portion of the western army.

These defections and acts of inaction left the remaining western forces overwhelmed. By 2:00 pm, the western coalition had shattered, allowing Ieyasu to consolidate his power and eventually be named shōgun in 1603.1