Mitsunari Ishida (1559-6 November 1600) was the daimyo of the Ishida clan and a former vassal of the Toyotomi clan. Loyal to Hideyoshi Hashiba, Mitsunari commanded the Western Army loyalists defending Hideyoshi's realm after his death against the Tokugawa clan. He died in battle in the Battle of Sekigahara.
Mitsunari Ishida was born in Omi Province, the second son of Masatsugu Ishida. The Ishida were vassals of the Azai clan and after 1573 became vassals of the Oda clan. While Hideyoshi Hashiba was the daimyo of Nagahama, Mitsunari met him at a young age, having been a monk at a Buddhist temple. He aided him in the sieges of Tottori and Takamatsu and after the death of Nobunaga Oda in 1582, he became the leading strategist of Hideyoshi and along with Kiyomasa Kato and Masanori Fukushima was called one of his "children".
His first major battle was the Battle of Yamazaki, where he captured Mt. Tenno and used it as a vantage point from which to fire rifles and throw boulders at the Akechi army. After the battle of Yamazaki, he became one of the most trusted generals of Hideyoshi and fought again in the Battle of Shizugatake in 1583 and Battle of Komaki-Nagakute in 1584, where he planned out the piecemeal destruction of the Tokugawa army. His greatest moment was his capture of Otsu Castle in 1590, defeating a flood attack by the Hojo clan and capturing the castle, helping the Toyotomi army to capture the main city of Odawara. After the campaigns in Japan for Hideyoshi's unification, he engaged in the Japanese invasions of Korea from 1592-1593 and 1597-1598. The Japanese forces returned home when Hideyoshi died of illness.
When Hideyoshi died, his son Hideyori Toyotomi was made the Shogun at the age of only nine. Mitsunari was one of the Go-Bugyo regents who was responsible for taking care of him, but an anti-Ishida coalition built around the Fukushima clan. Masanori Fukushima was his former fellow general, but did not appreciate Mitsunari's arrogance and rivalry with him, and wanted to assassinate him. In an attack on Kyoto, the Anti-Ishida Coalition of many Toyotomi retainers (including Kiyomasa Kato, Terumasa Ikeda, and Tadaoki Hosokawa) was repelled by Mitsunari's loyalists and the Tokugawa clan, while Mitsunari was hiding.
With Masanori defeated, Mitsunari had Ieyasu Tokugawa as his new rival. Mitsunari was persuaded by his advisors to send a declaration of war to Tokugawa, and Mitsunari rallied the Uesugi, Mori, Shimazu, Chosokabe, Tachibana, and other western clans to join his pro-Toyotomi Western Army. He engaged in a series of battles such as Otsu Castle, Ishigaki Plains, Fushimi Castle, and Kuzegawa. Mitsunari's allies won most of the early battles, but in the Battle of Sekigahara in November 1600, he faced a misadvantage: most of the battlefield was obscured by fog and many of his allies were demoralized. Yoshihiro Shimazu refused to fight because Mitsunari's messenger disrespected him by refusing to dismount his horse, and Hideaki Kobayakawa was disloyal to Hideyoshi ever since Hideyoshi abandoned him, his adopted son, at a young age. Hideaki defected, causing Mitsunari to lose his advantage in cannon and muskets. He retreated, and was defeated in the chaos of the battle that followed.