Battle of Okehazama
Also called: Battle of Dengakuhazama
Oda Nobunaga of Owari fell on the invading army of Imagawa Yoshimoto at Okehazama and killed Yoshimoto. The victory made Nobunaga’s name and let Tokugawa Ieyasu, until then bound to the Imagawa, strike out on his own.
Utagawa Toyonobu · Public domain · Source
In 1560 Imagawa Yoshimoto, lord of Suruga, Tōtōmi and Mikawa, invaded Owari with an army said to number around 25,000. While his vanguard under Matsudaira Motoyasu (later Tokugawa Ieyasu) took Oda forts, Yoshimoto halted near Okehazama.
On 12 June 1560 Nobunaga, with a few thousand men, used a downpour to strike Yoshimoto’s headquarters, and Yoshimoto was killed on the spot. The attack was long told as a surprise attack by a roundabout route, though some historians argue it was a frontal assault. Nobunaga went on to secure Owari and push into Mino and Kyoto, while Ieyasu returned to Okazaki, became independent and allied with Nobunaga in 1562 (the Kiyosu alliance). The Imagawa declined and fell in 1569.