MARE ISLAND — It was the fear, in the spring of 1861, that pro-Confederate secessionists had designs on taking over, or at least disrupting, the Union’s shipyard at Mare Island, which brought the Marine Corps there. “Possession of Mare Island almost ensured control of the Pacific, because there was located the only dock large enough to handle ocean-going ships,” wrote Lt. Cmdr. Arnold Lott in his history of Mare Island. Newly established shipyard commander Capt.