Battle of Saipan

The Battle of Saipan was an amphibious assault launched by the United States against the Empire of Japan during the Pacific campaign of World War II between 15 June and 9 July 1944. The initial invasion triggered the Battle of the Philippine Sea, which effectively destroyed Japanese carrier-based airpower, and the battle resulted in the American capture of the island. Its occupation put the major cities of the Japanese home islands within the range of B-29 bombers, making them vulnerable to strategic bombing by the United States Army Air Forces. It also precipitated the resignation of Hideki Tōjō, the prime minister of Japan.

Saipan was the first objective in Operation Forager, the campaign to occupy the Mariana Islands that got underway at the same time the Allies were invading France in Operation Overlord. After a two-day naval bombardment, the U.S. 2nd Marine Division, 4th Marine Division, and the Army's 27th Infantry Division, commanded by Lieutenant General Holland Smith, landed on the island and defeated the 43rd Infantry Division of the Imperial Japanese Army, commanded by Lieutenant General Yoshitsugu Saitō. Organized resistance ended when at least 3,000 Japanese soldiers died in a mass gyokusai attack, and afterward about 1,000 civilians committed suicide.

The capture of Saipan pierced the Japanese inner defense perimeter, and forced the Japanese government to inform its citizens for the first time that the war was not going well. The battle claimed more than 46,000 military casualties and at least 8,000 civilian deaths. The high percentage of casualties suffered during the battle influenced American planning for future assaults, including the projected invasion of Japan.

Up to early 1944, Allied operations against the Japanese military in the Pacific were focused on securing the lines of communication between Australia and the United States. These operations had recaptured the Solomon Islands, eastern New Guinea, western New Britain, the Admiralty Islands, and the Gilbert and Marshall Islands.

To defeat Japan, Admiral Ernest J. King, Commander in Chief, United States Fleet, sought to execute War Plan Orange, which the Naval War College had been developing for four decades in the event of a war. The plan envisioned an offensive through the Central Pacific that originated from Hawaii, island-hopped through Micronesia and the Philippines, forced a decisive battle with the Japanese Navy, and brought about an economic collapse of Japan.

As early as the Casablanca Conference in January 1943, King presented the case to the Combined Chiefs of Staff for an amphibious offensive in the Central Pacific – including the Marshall Islands and Truk – that would capture the Mariana Islands. He stated that the occupation of the Marianas – specifically Saipan, Tinian and Guam – would cut the sea and air route from the Japanese home islands to the western Pacific, but the Combined Chiefs of Staff made no commitment at the time. General Douglas MacArthur, Supreme Commander of Allied Forces in the Southwest Pacific Area, objected to King's proposed Central Pacific offensive. He argued that it would be costly and time-consuming and would pull resources away from his drive in the Southwest Pacific toward the Philippines.

At the Quebec Conference in August 1943, King continued to advocate for including the Marianas in a Central Pacific offensive. He suggested that the strategic importance of the Marianas could draw the main Japanese fleet out for a major naval battle. King's advocacy gained support from General Henry H. Arnold, Chief of the Army Air Forces, who wanted to use the newly developed B-29 bomber. The Marianas could provide secure airfields to sustain a strategic bombing offensive as the islands put much of Japan's population centers and industrial areas within the B-29's 1,600 mi (2,600 km) mile combat radius. At the Cairo Conference in November 1943, the Combined Chiefs of Staff supported both MacArthur's offensive in the Southwest Pacific and King's in the Central Pacific, adding the Marianas as an objective for the Central Pacific offensive and setting 1 October 1944 as the date for their invasion.

Admiral Chester Nimitz, Commander-in-Chief, Pacific Ocean Areas led the Central Pacific offensive. In January–February 1944, the Marshall Islands were quickly captured and a massive American carrier-based air attack on Truk demonstrated that it could be neutralized and bypassed. On 12 March 1944, the Joint Chiefs of Staff moved the date of the invasion up to 15 June with the goal of creating airfields for B-29s and developing secondary naval bases. Nimitz updated the plans for the Central Pacific offense, codenamed Granite II, and set the invasion of the Marianas, codenamed Forager, as its initial objective. Saipan would be the first assault.