"Sōhei"|僧兵|literally "monk warriors" were Buddhist warrior monks of feudal Japan. At certain points of history they held considerable power, obliging the imperial and military governments to collaborate.
They were similar to the mountain ascetic "yamabushi", but unlike the solitary "yamabushi", "sōhei" generally organized into large armies or mobs. A famous monastery is the Enryaku-ji on Mount Hiei, just outside Kyoto.
The "sōhei" shared many similarities with the Western lay brothers, members of a monastic order who might not have been ordained.
Warrior monks first appeared in a significant way in Japan in the middle of the 10th century, when bitter political feuds began between different temples, different sub-sects of Buddhism, over imperial appointments to the top temple positions (abbot, or "zasu"). Much of the fighting over the next four centuries was over these sorts of political feuds, and centered around the temples of Kyoto and Nara, namely the Tōdai-ji, Kōfuku-ji, Enryaku-ji, and Mii-dera, the four largest temples in the country.
The first armed conflict broke out in 949, when 56 monks from Tōdai-ji staged a protest at the residence of a Kyoto official, over an appointment that displeased them. Protests of this sort continued through the 10th century, often breaking out into brawls in which some participants would be killed. In 970, following a dispute between Enryaku-ji and the Yasaka Shrine of Kyoto, the former established the first standing army of warrior monks. It is not entirely clear whether or not this standing army consisted of monks from Enryaku-ji, or were more like mercenaries, since Ryōgen, the abbot who established this army, also established a code of monastic conduct that prevented monks from leaving Mount Hiei during their 12-year training, from covering their faces, and from carrying weapons.