The civil war that brought an end to Japan's fabled Heian era and initiated the Kamakura Bakufu or military govenment.
The war was fought between the Taira and Minamoto clans, the former ensconced in Kyoto, where they had managed since the mid-twelfth century to supplant the Fujiwara, and the latter establishing themselves in the East Country.
The war itself began in 1180 when Minamoto no Yorimasa, the only high-ranking Minamoto left in the noble ranks in Kyoto, rebelled against Taira no Kiyomori. Though soon overcome, his example inspired his cousin Minamoto no Yoritomo, living in the East Country, to raise an army to chastise the Taira, whom he painted as usurpers. At first, Yoritomo was defeated by superior forces; but in time he was able to forge alliances with a number of other military men and create an army sizeable enough to drive the Taira out of the capital.
Fleeing westward down the Inland Sea, the Taira resisted valiantly in a series of battles on land and sea, but in the end, at Dan no Ura on the southern tip of Honshu, they were overcome. The young Emperor Antoku, a grandson of Taira no Kiyomori, was drowned, and most of the leaders of the Taira clan--all of them courtiers with high titles--were either killed in battle or executed. The Minamoto alliance had triumphed.
Of course, there had been battles between warriors rivals before, even in the same century. What made the Genpei War so significant was that the victors, largely in the process of creating an effective alliance between great regional powers, had established policies and institutions that actually superseded aristocratic rule. Rather than simply usurping the old courtly offices or attaching themselves to aristocratic patrons, the Minamoto created new offices and appointed to them warriors whose allegiance was to the Minamoto clan first of all. Among these were stewards and constables with the authority to adjudicate in matters both economic and political. Of course, the new system was not stronger than the alliances on which it rested; but there can be no denying that it effectively challenged the power of the court in the most fundamental of ways.
The long-term implications of the shift in administrative control were perhaps not immediatly obvious to either the old nobility or the military aristocracy. Certainly, the Minamoto and their successors, the Hojo, made no attempt to eliminate the court altogether. On the contrary, Yoritomo and his men were rather generous with many noble families, whose prestige in cultural matters they recognized. And in many other ways, one can argue that courtly values continued to be a powerful cultural and political force. For example, the Lady Daibu was recalled to court and her poetry published in an anthology and the poet Fujiwara no Teika flourished at this time.
Finally, though, the Genpei War did leave the noble families reduced in influence. For the rest of the medieval period, they would have to share power with a new military aristocracy that had traditions and values of its own, some of which were not entirely compatible with those of the court families.
The Kamakura era's history became more and more turbulent as these pressures increased.