D-Day for Saipan, headquarters of Japan's Central Pacific Fleet, was June 15, 1944. The island's Japanese defenders numbered almost 30,000. While the Navy landed 8,000 Marines from the 2nd and 4th divisions in the first 20 minutes of the assault, Japanese shoreline defenses kept the invasion force fighting for three days to protect the beachhead.

Naval bombardment on the first day was insufficient, although it improved to the point that one Japanese officer credited it as the single greatest factor in the American victory. Four battleships, five cruisers and 39 destroyers provided on-call fire, with destroyers taking out enemy gun positions and the larger ships assigned to saturation bombardment. Gunboats, converted infantry landing craft, were especially effective against the caves because their shallow draft allowed them to move in close to shore. Illumination from Navy-launched star shells helped the Marines protect their lines at night.

Saipan's mountainous terrain made seizure of the approximately 14-mile-long by 6-mile-wide island an arduous process for the Marines and the 27th Infantry Division. Prolonged hand-to-hand combat characterized the campaign. In addition to the fierce enemy resistance and the logistical problems caused by the sheer size of the U.S. force, the mix of unfamiliar Army and Marine units made coordinating their movements difficult.

From their original beachheads on the island's southwest coast, it took the 4th Division Marines until July 9 to reach Marpi Point at Saipan's northern tip. There, the remaining defenders and a number of Japanese civilians jumped from the high cliffs rather than surrender; estimates of the total number of civilians who killed themselves on Saipan run as high as 22,000. All but a thousand of the Japanese defenders died in battle or by their own hands. U.S. casualties on Saipan were 16,612; 13,000 were Marines. Due to the difficulties on Saipan, the invasion of Guam, scheduled for June 18, was postponed until late July.