The Onin War is a civil war during the Muromachi period in Japan.

A dispute between Hosokawa Katsumoto and Yamana Sōzen escalates into a nationwide war involving the Ashikaga shogunate and a number of daimyo in many regions of Japan.The war initiates the Sengoku jidai, "the Warring States Period", a long, drawn-out struggle for domination by individual daimyo, resulting in a mass power-struggle between the various houses to dominate the whole of Japan.

It is during this time, though, that there emerges three individuals who will later be considered the three great daimyo of the Sengoku Period, and who will eventually unite Japan under one rule; they are Oda Nobunaga, Toyotomi Hideyoshi, and Tokugawa Ieyasu.