In the campaigns of 1943 and the first half of 1944, the Allies had captured the Solomon Islands, the Gilbert Islands, the Marshall Islands and the Papuan Peninsula of New Guinea. This left the Japanese holding the Philippines, the Caroline Islands, Palau Islands and Mariana Islands.
It had always been the intention of the American planners to bypass the Carolines and Palaus and to seize the Marianas and Taiwan. From these latter bases communications between the Japanese homeland and Japanese forces to the south and west could be cut. In addition, from the Marianas Japan would be well within the range of an air offensive relying on the new Boeing B-29 Superfortress long-range bomber with its operational radius of 1,500 mi (2,400 km).
While not part of the original American plan, Douglas MacArthur, commander of the Southwest Pacific Area command, obtained authorization to advance through New Guinea and Morotai toward the Philippines. This allowed MacArthur to keep his personal pledge, made in his “I shall return” speech, to liberate the Philippines, and also allowed the active use of the large forces built up in the southwest Pacific theatre. The Japanese, expecting an attack somewhere on their perimeter, thought an attack on the Caroline Islands most likely. To reinforce and supply their garrisons, they needed naval and air superiority, so Operation A-Go, a major carrier attack, was prepared for June 1944.
Bombardment of Saipan began on 13 June 1944. Fifteen battleships were involved, and 165,000 shells were fired. Seven modern fast battleships delivered twenty-four hundred 16 in (410 mm) shells, but to avoid potential minefields, fire was from a distance of 10,000 yd (9,100 m) or more, and crews were inexperienced in shore bombardment. The following day the eight older battleships and 11 cruisers under Admiral Jesse B. Oldendorf replaced the fast battleships but were lacking in time and ammunition.
The landings began at 07:00 on 15 June 1944. More than 300 LVTs landed 8,000 Marines on the west coast of Saipan by about 09:00. Eleven fire support ships covered the Marine landings. The naval force consisted of the battleships Tennessee and California. The cruisers were Birmingham and Indianapolis. The destroyers were Norman Scott, Monssen, Colahan, Halsey Powell, Bailey, Robinson and Albert W. Grant. Careful Japanese artillery preparation — placing flags in the lagoon to indicate the range — allowed them to destroy about 20 amphibious tanks, and the Japanese strategically placed barbed wire, artillery, machine gun emplacements, and trenches to maximize the American casualties. However, by nightfall the 2nd and 4th Marine Divisions had a beachhead about 6 mi (10 km) wide and 0.5 mi (1 km) deep. The Japanese counter-attacked at night but were repulsed with heavy losses. On 16 June, units of the U.S. Army’s 27th Infantry Division landed and advanced on the airfield at Ås Lito (which is now the location of Saipan International Airport). Again the Japanese counter-attacked at night. On 18 June, Saito abandoned the airfield.
The invasion surprised the Japanese high command, which had been expecting an attack further south. Admiral Soemu Toyoda, commander-in-chief of the Japanese Navy, saw an opportunity to use the A-Go force to attack the U.S. Navy forces around Saipan. On 15 June, he gave the order to attack. But the resulting battle of the Philippine Sea was a disaster for the Imperial Japanese Navy, which lost three aircraft carriers and hundreds of planes. The garrisons of the Marianas would have no hope of resupply or reinforcement.
Without resupply, the battle on Saipan was hopeless for the defenders, but the Japanese were determined to fight to the last man. Saito organized his troops into a line anchored on Mount Tapotchau in the defensible mountainous terrain of central Saipan. The nicknames given by the Americans to the features of the battle — “Hell’s Pocket”, “Purple Heart Ridge” and “Death Valley” — indicate the severity of the fighting. The Japanese used the many caves in the volcanic landscape to delay the attackers, by hiding during the day and making sorties at night. The Americans gradually developed tactics for clearing the caves by using flamethrower teams supported by artillery and machine guns.
The operation was marred by inter-service controversy when Marine General Holland Smith, unsatisfied with the performance of the 27th Division, relieved its commander, Army Major General Ralph C. Smith. However, General Holland Smith had not inspected the terrain over which the 27th was to advance. Essentially, it was a valley surrounded by hills and cliffs under Japanese control. The 27th took heavy casualties and eventually, under a plan developed by General Ralph Smith and implemented after his relief, had one battalion hold the area while two other battalions successfully flanked the Japanese.
By 7 July, the Japanese had nowhere to retreat. Saito made plans for a final suicidal banzai charge. On the fate of the remaining civilians on the island, Saito said, “There is no longer any distinction between civilians and troops. It would be better for them to join in the attack with bamboo spears than be captured.” At dawn, with a group of 12 men carrying a great red flag in the lead, the remaining able-bodied troops — about 3,000 men — charged forward in the final attack. Amazingly, behind them came the wounded, with bandaged heads, crutches, and barely armed. The Japanese surged over the American front lines, engaging both army and marine units. The 1st and 2nd Battalions of the 105th Infantry Regiment were almost destroyed, losing 650 killed and wounded. However, the fierce resistance of these two battalions, as well as that of Headquarters Company, 105th Infantry, and supply elements of 3rd Battalion, 10th Marine Artillery Regiment resulted in over 4,300 Japanese killed. For their actions during the 15-hour Japanese attack, three men of the 105th Infantry were awarded the Medal of Honor — all posthumously. Numerous others fought the Japanese until they were overwhelmed by the largest Japanese Banzai attack in the Pacific War.
By 16:15 on 9 July, Admiral Turner announced that Saipan was officially secured. Saito — along with commanders Hirakushi and Igeta — committed suicide in a cave. Also committing suicide at the end of the battle was Vice-Admiral Chuichi Nagumo — the naval commander who led the Japanese carriers at Pearl Harbor and Midway — who had been assigned to Saipan to direct the Japanese naval air forces based there.
In the end, almost the entire garrison of troops on the island — at least 30,000 — died. For the Americans, the victory was the most costly to date in the Pacific War. 2,949 Americans were killed and 10,464 wounded, out of 71,000 who landed. Hollywood actor Lee Marvin was among the many American wounded. He was serving with “I” Company, 24th Marine Regiment, when he was shot in the buttocks by Japanese machine gun fire during the assault on Mount Tapochau. He was awarded the Purple Heart and was given a medical discharge with the rank of Private First Class in 1945.