Cloistered Emperor
In 1180, Prince Mochihito, a son of Cloistered Emperor Go-Shirakawa, made a national call to arms of the Minamoto clan all over Japan to rebel against the Taira. Yoritomo took part in this, especially after things escalated between the Taira and Minamoto after the death of Minamoto no Yorimasa and Prince Mochihito himself.