Complex sentence: Many of the African American shipyard laborers who had migrated from the Southern U.S. decided to remain in Marin City, and they became the core of the community when the other guest workers departed at the end of the war.
Split into simpler sentences: Many of the African American shipyard laborers who had migrated from the Southern U.S. decided to remain in Marin City. They became the core of the community when most of the guest laborers departed at the end of the war.