The capture of Saipan was considered essential for the establishment of airfields which would accommodate the B-29 Superfortress bombers to be used in what was then the planned full-scale invasion of the Japanese mainland. On June 15, 1944, an armada of 535 ships, carrying 127,570 U.S. military personnel which included Marines from the 2nd and 4th Marine Divisions, began the invasion of Saipan. Japanese soldiers seldom surrendered during World War II and, as the American military invasion went badly for the Japanese, they were ordered by their superiors on Saipan to kill seven American Marines or soldiers for every soldier they lost, or commit suicide.