head of the most powerful warrior clan in western Japan in the 15th century.

Yamana's attempts to increase his family's rank and influence brought him into conflict with a rival clan in eastern Japan and resulted in the Ōnin War (1467–77), which was followed by a century of internecine strife. As a Buddhist monk, he took the name Sōzen, and, because of his quick temper and scarlet complexion, he was sometimes called Aka-nyūdō, the Red Monk.

Yamana's rival for power within the central government, or shogunate, was Hosokawa Katsumoto, head of an important coalition of warriors from eastern Japan, and the kanrei, or shogunal prime minister. The conflict between the two contenders for power erupted into warfare in 1467, when the shogun, Ashikaga Yoshimasa (ruled 1449–73), attempted to name his infant son, rather than his younger brother, as his heir.

Although both Yamana and Hosokawa died in 1473, the fighting dragged on for four more years, when it ended in a stalemate. By that time the last vestiges of the central government's control over the outlying regions of Japan had been eliminated, and local warrior families had begun to quarrel among themselves. Within the central government the Yamana family lost out to the Hosokawa.