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.

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.

The high percentage of casualties suffered during the battle influenced American planning for future assaults, including the projected invasion of Japan.

[10] 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.

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,[12] but the Combined Chiefs of Staff made no commitment at the time.

[17] 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.

[18] 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.

[23] 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.

[27] The Marianas were considered particularly important to protect as their capture would put Japan within bombing range of the B-29 bomber[28] and allow the Americans to interdict the supply routes between the Japanese home islands and the western Pacific.

[29] The Imperial Japanese Navy planned to hold the defense line by defeating the United States fleet in a single decisive battle,[30] after which, the Americans were expected to negotiate for peace.

[35] As part of this plan the Japanese could deploy over 500 land-based planes – 147 of them immediately in the Marianas – that made up the 1st Air Fleet under the command of Vice Admiral Kakuji Kakuta whose headquarters was on Tinian.

[46] This area was filled with caves and ravines concealed by forest and brush,[47] and the mountainous terrain would force tanks to stay on the island's few roads, which were poorly constructed.

[54] Saipan was the first island during the Pacific War where the United States forces encountered a substantial Japanese civilian population,[55] and the first where U. S. Marines fought around urban areas.

[62] The invasion fleet, consisting of over 500 ships and 300,000 men,[c] got underway days before the Allied forces in Europe invaded France in Operation Overlord on 6 June 1944.

If an invading force broke through the beach defenses, there was no organized fallback position: the Japanese troops would have to rely on Saipan's rough terrain, especially its caves, for protection.

Much of the building material sent to Saipan, such as concrete and steel, had been sunk in transit by American submarines,[41] and the timing of the invasion surprised the Japanese, who thought they had until November to complete their defense.

[111] The beaches were fortified by trenches and a few pillboxes,[112] but the landings were mainly contested by constant and intensive fire by Japanese artillery, mortars,[113] and machine guns.

[121] The bridgehead was only about two-thirds the size of the planned objective,[122] the two Marine divisions were separated by a wide gap just north of Charan Kanoa,[123] and the Japanese artillery remained intact on the high ground surrounding the beach.

[127] All the attacks were repulsed,[128] partly by the firepower provided by the tanks and artillery that had been unloaded during the day as well as by American warships that illuminated the combat areas with star shells.

[157] Less than a half hour after the start of the amphibious invasion,[158] he announced the implementation of Operation A-Go,[159] the Japanese Navy's current plan to destroy the American fleet.

"[162] Originally, the Japanese Navy sought to have the battle take place in the Palaus or Western Carolines,[163] and MacArthur's invasion of Biak had led them to believe that they could lure the American fleet there.

[183] It held most of the island's high ground, which allowed the Japanese to observe American movements, and the rough terrain was filled with caves concealed by brush.

[201] Spotters flying in L-4 Grasshoppers helped direct ground artillery,[202] and Navajo code talkers relayed information about Japanese troop movements.

[217] Igeta's 31st Army Headquarters sent a telegram from the island on 27 June, stating the Japanese would not be able to hold due to the American preponderance in artillery, sea and air power, as well of a lack of equipment and supplies, including food and water.

[225] Saitō intended to form a new line in northern Saipan that would be anchored on Tanapag on the west, running southeast to a village called Tarahoho, and through to the east coast.

[229] As Saitō's attempt to form the defense line collapsed,[230] he eventually moved his final headquarters near Makunsha village on the west coast north of Tanapag.

[234] On 5 July, the 27th Infantry Division encountered strong resistance in a narrow canyon on the east coast north of Tanapag that they dubbed "Harikari Gulch", which expanded into a two-day battle.

[242] On 6 July, Saitō decided the situation was hopeless and sent out orders for the remainder of his forces to perform gyokusai, one final suicide attack to destroy as many of the enemy as possible.

[243] He set the attack for the following day to give the troops a chance to concentrate what was left of his forces and put his divisional chief of staff, Colonel Takuji Suzuki,[244] in charge.

[313] The Army Air Force was confident that strategic bombing could destroy Japan's military production and that the Marianas provided excellent airbases for doing so because they were 1,200 mi (1,900 km) miles from the Japanese home islands.