How did Kobayakawa Hideaki’s actions affect 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.

At the Battle of Sekigahara, Kobayakawa Hideaki’s actions were the decisive turning point that secured victory for Tokugawa Ieyasu.

Although Ishida Mitsunari had positioned Hideaki on Mount Matsuo to flank the eastern army, Hideaki had secretly pledged his support to Ieyasu. When the battle began, Hideaki initially remained inactive, prompting Ieyasu to fire upon his position to force a decision. Shortly after noon, Hideaki betrayed the western coalition by leading his 15,000 men down the mountainside to attack the Otani Yoshitsugu lines.

This betrayal caused a collapse in the western army's defenses, as other divisions followed suit and defected. The resulting chaos forced the western forces to retreat, effectively ending the conflict in Ieyasu's favor. Following the battle, Ieyasu rewarded Hideaki with redistributed lands for his pivotal role in the victory.1