Minamoto Yoritomo (1147
), head of the Minamoto
clan that defeated the Taira
family in the Genpei war
), fundamentally altered the power structure in Japan and established what became known as the Kamakura era
After Emperor Goshirakawa brought in the provincial military clan houses of Taira and Minamoto into the capital, Heiankyo (now Kyoto), in his struggle against retired emperor Sutoku (the Hogen wars, 1156), the Taira and Minamoto were engaged in a number of intense power struggles.
In the Heiji Wars (1159), Emperor Goshirakawa sided with Taira Kiyomori to defeat Minamoto Yoshitomo. In this battle, the Taira family won a complete victory, and in a moment of compassion, Taira Kiyomori allowed Minamoto Yoshitomo's sons to escape execution. One of these sons was Yoritomo.
Yoritomo grew up in Eastern Japan. There he amassed opposition to the Taira, eventually waging full-scale war against them.
With a series of military victories, Yoritomo and his brother Minamoto Yoshitsune eventually obliterated the Taira in the Genpei war of 1185. Now in control of the capital, Yoritomo set about consolidating power into his own hands. Since he was not in a position to use the cloistered emperor power structure that had typified the late Heian era, he devised a power structure that did not compromise the existing order, but allowed him to exert administrative power over the country.