The Yamana family had seen many defeats over the years, while the Hosokawa family was one of the three families which controlled the position of Kanrei, deputy to the Shogun. Thus, Yamana Sōzen resented the wealth and power enjoyed by his son-in-law, Hosokawa Katsumoto. Unwilling to engage him in open warfare until he was sure of his strength, Yamana chose to intervene in a number of succession disputes and other political affairs, thwarting Hosokawa's plans and desires, and slowly gaining allies for himself.
In 1464, a succession dispute erupted over the shogunate itself. The Shogun, Ashikaga Yoshimasa, was considering retiring. Hosokawa supported the Shogun's brother, Ashikaga Yoshimi as the successor, and so Yamana chose to support Ashikaga Yoshihisa, the Shogun's infant son. In 1466, both sides having spent several years gathering forces, both Yamana and Hosokawa felt ready to engage the other, and skirmishes began to break out.
In 1467, the first year of Ōnin by Japanese reckoning, both men began to prepare more seriously for the coming conflict; they sought safehouses and planned for fighting in the streets. Yamana took Yoshimi to the Shogun's residence, where Hosokawa, who supported Yoshimi's claim to the shogunate, could not get at him. He was essentially a hostage.
Seeing that open war in the capital would spread to the provinces, the Shogun declared that the first to make an attack within the city would be labeled a rebel against the Shogunate, and enemy of the state. Thus, for several months, the conflict quieted, neither side willing to make a move. Finally, in March of 1467, the home of a Hosokawa officer was destroyed by fire. After several more minor attacks and political maneuvers, in May, Hosokawa attacked outright the mansion of one of Yamana's generals. Nevertheless, Yamana, not Hosokawa, was labeled a rebel, and enemy of the state. Some of Yamana's followers deserted, joining Hosokawa's morally superior side, but many more switched sides as a result of the work of Hosokawa's emissaries to the provinces where Yamana and his allies drew their armies.
By New Year's of 1468, nearly a year since the war began, the fighting tapered off. For much of that year, the two forces engaged in glaring contests and limited sorties, both desiring to rebuild and to act only defensively. Both spent the next several years in political, not military, conflict, and in 1469, the Shogun named Yoshihisa, his son, to be his heir. But Hosokawa was weary of battle, and wished for peace. Peace was had, and a few years later, in 1473, both Hosokawa and Yamana died.