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Why did Ishida Mitsunari oppose Tokugawa Ieyasu?

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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.

Ishida Mitsunari opposed Tokugawa Ieyasu primarily because he feared that Ieyasu intended to usurp power from the young Toyotomi Hideyori, the son and heir of the late ruler Toyotomi Hideyoshi.12

Following Hideyoshi’s death in 1598, Ieyasu—one of five regents appointed to protect the young heir—began consolidating his influence through strategic political marriages and by relocating to Fushimi Castle. These actions alarmed other regents and daimyō, including Ishida, who viewed them as clear signs that Ieyasu sought to supplant the Toyotomi line.2

In response, Ishida Mitsunari formed a coalition of loyalist daimyō to reassert the authority of the Toyotomi clan and protect the heir. This opposition escalated from political maneuvering and a failed assassination attempt to open conflict, culminating in the Battle of Sekigahara in 1600, where Ishida’s forces were defeated by Ieyasu’s army.21