Zipang: Shinsou Kairyuu

This story begins around the time of the Heiji Rebellion (1159–1160), with the conflict between the Genji (Minamoto clan), led by Yoshitomo, and the Heike (Taira clan), led by Kiyomori. Yoshitomo's third son, 13-year-old Yoritomo, goes into battle against the Taira clan alongside his father and two older brothers. The events of this disastrous battle will change the course of Yoritomo's life, and eventually lead to his becoming a prominent figure in the history of the period.

(Source: MangaHelpers)