The Ônin War, or Ônin disturbance (so called because it started in the Imperial era of Ônin), was a bloody affair, lasting 11 years, which is generally considered to have opened the Sengoku Period.
While the direct cause of the war was a conflict between the Hosokawa and Yamana families, there were many other contributing factors. For one, the central authority of the Ashikaga Shogunate had waned. The provinical shugo no longer controlled the provinces, having ceded power to the new daimyo families, who now controlled most of the military resources. The current shogun, Ashikaga Yoshimasa, had in fact grown tired of the day-to-day duties of governance, wishing to abdicate so that he could leave public office and concentrate on his artistic endeavors.
The shogun did, however, retain the important authority to declare someone responsible for rebellion against the Imperial court. Even though most of the bakufu's power had waned, this one act could still tip the scales in an armed conflict against the 'rebel'.
Fighting in the Capital
Although there were incidences back and forth between the two families, actual fighting opened up at the end of May, with Hosokawa troops attacking a mansion in the city of one of Yamana's generals, Isshiki. Fighting went on for days, despite Yoshimasa's attempts to order a truce. The fighting was mostly close quarters fighting, with trenches and barricades built up in the city.
By early July, Yoshimasa had branded Yamana Sozen and his allies as rebels, despite the fact that the Hosokawa had started the fighting in the capital. He charged his brother, Yoshimi, with subduing the rebels, appointing Hosokawa Katsumoto as his commanding general. This removed some of the Yamana's support. In addition, the Hosokawa were able to stir up disturbances in their adversaries' territories--including those of the Ouchi and Shiba--forcing them to send troops back to defend their own territories. Sozen sent to Harima for reinforcements, which made their way to Tanba, and from their fought their way to the city.
In July, Hosokawa forces (known as the Eastern Army) controlled the main buildings of the Bakufu, the Jiso-In, Shokokuji, and the Hosokawa's own mansion. Yamana (the Western Army), held territory to the south and west, running a line east from Itsutsuji-Omiya, with their main encampment at moder day Nishijin
Fierce fighting continued until 1468, eventually settling into a prolonged military occupation, but not before destroying the capital. Armed conflicts in the capital lessened after that, moving to politics, although fierce fighting continued out in the provinces. In a strange turn of events, Yoshimi found himself a general for the Yamana, turning the conflict between the two families into an even larger conflict between the shogun and his brother.
- Sansom, George. A History of Japan 13334-1615. Stanford: Stanford University Press, 1963.