The founder Kiyohira stayed in the Toyota fortress for a while before moving his headquarters to Hiraizumi. He crossed over the Koromogawa River, and beyond the southern border of the six counties in the beginning of the 12th century. According to the Azuma kagami, Kiyohira ruled the domain from the Shirakawa barrier in the south to the Sotogahama coast in the north and controlled over 10,000 villages. He grew into the chief of warriors in the region of Mutsu. He gained more power than other previous local warriors had before. He controlled the military and trade in the region.
It seems that the authority over the southern domain beyond Hiraizumi was not so great. Motohira, the second generation of the Oshu Fujiwara family, was appointed Oryo shi (police conductor) of six counties and Dewa. He attempted to hold authority over Mutsu and Dewa by forming connections with aristocracy like Fujiwara no Motonari who served the cloistered emperor. Thus the Oshu Fujiwara family controlled the provincial government.
Hidehira, the third generation of the Oshu Fujiwara family, was appointed General and gained military power in this district in 1170. Furthermore, he was appointed the governor of Mutsu. This was the first time a local warrior had become governor and he reigned over the Tohoku District independently. During the reign of Hidehira, Hiraizumi was most prosperous.
The war between the Minamoto and the Taira family broke out in the same period. Minamoto no Yoshitsune who had exerted his tactical genius in the campaign against the Taira family, escaped from his brother Yoritomo to Hiraizumi, hoping for Hidehira's protection. He left his will to his son Yasuhira, the fourth generation of the Oshu Fujiwara family, that he ought to serve Yoshitsune as a vassal. But Yasuhira broke the will and attacked Yoshitsune. Yoshitsune killed himself in 1189. Yoritomo set forth from Kamakura, with a large force, on the pretext that Yasuhira gave shelter to Yoshitsune.
Yasuhira set fire to the Hiraizumi no tachi (his manor), after being informed of defeat. He fled to Hokkaido. Yoritomo's army joined with the army from Dewa at Jingaoka, Shiwa county. The total number of these warriors reached two hundred eighty thousand. Finally Yasuhira was killed by his vassal Kawada Jiro. Thus, ｔhe Oshu Fujiwara family died out.
