The Battle of Koromo River took place during the opening years of the Kamakura period (12th century) of Japan. After the destruction of the Heike, Minamoto no Yoshitsune conflicted with his brother Minamoto no Yoritomo, and fled into Hiraizumi, Mutsu Province. He was sheltered by Northern Fujiwara's 3rd ruler Fujiwara no Hidehira. Hidehira appointed Yoshitsune as general to be opposed to Yoritomo, but he died of illness on October 29, 1187.