Battle of Dan-no-ura
The Hōjō were an offshoot of the Minamoto's arch-enemy, the Taira of the Kammu branch, originating in Izu Province. They gained power by supporting the extermination of the Taira by intermarrying with and supporting Minamoto no Yoritomo in the Battle of Dan-no-ura. Just 18 years after, the Hōjō usurped power with Yoritomo's death.