Complex sentence: Yoritoki and the Abes came into conflict with the Minamoto clan as the Minamotos began to expand their holdings north into Abe territory and the Abes began to raid territories south of their border.
Split into simpler sentences: Yoritoki and the Abes came into conflict with the Minamoto clan as it began to expand north into Abe territory. The Abe clan began to raid territories south of their border.