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Mariana Islands

Digital relief map of Mariana Islands

The Marianas Islands are an island arc located about 1400 miles (2250 km) south of Japan. There are 15 islands in the chain, which extends about 425 miles (684 km) from Farallon de Pajaros in the north to Guam in the south. Most are mountainous, with elevations up to 2585' (788 meters).  The islands have a total area of 402 square miles (1041 km2), of which over half (225 square miles or 583 km2) is from Guam alone. The islands have relatively fertile soil and are covered with mixed scrub and grassland, with a few mangrove swamps. Beaches tend to be narrow and backed by coral cliffs and there are reefs off many of the shore lines.

Because they are located in the deep Pacific ocean basin, the islands have no sedimentary basement complex and no interesting mineral resources. However, they have a good climate for the cultivation of sugar cane, and supplied much of Japan's sugar. They are also strategically located along Japan's sea lanes to the Mandates.

In late 1941, the southernmost of the Marianas, Guam, had been a U.S. possession since the Spanish-American War of 1898. The remaining islands belonged to Japan, which had seized them from Germany in October 1914, during the First World War, and developed them for sugar production under the auspices of the South Seas Development Company. By the time war broke out in the Pacific, the Japanese population of the islands outnumbered the indigenous population (Chamorros) by two to one.

Rota, just north of Guam, produced enough sugar to support two refineries and a distillery and had a population of 764 Chamorros and 4800 Japanese and Koreans. A small airstrip was constructed on the north part of the island during the war. Other important islands include Saipan and Tinian.

The naval disarmament treaties specified that these islands were not to be fortified, but with the lapse of the treaties in the early 1930's Japan proceeded to build large airfields on Saipan, within easy range of Guam. The United States neglected the building of fortifications on Guam, which was considered too exposed to be held in the event of war. Japanese troops landed on Guam just two days after the attack on Pearl Harbor and easily conquered the island.

The Marianas Campaign: Operation FORAGER

The Marianas were identified as an important objective in prewar planning (Plan ORANGE), but it was not until August 1943, at the QUADRANT conference, that a formal decision was made to invade the Marianas following the seizure of the Palaus. In December 1943, at the SEXTANT conference, the U.S. Army Air Forces forcefully argued for an early invasion of the islands so that they could be used as bases for the strategic bombing of Japan by B-29 Superfortresses. Seizure of the Marianas would also open a number of options to the Allies, since bases here would be within range of Palau, the Philippines, Formosa, or the Bonins. Invasion of the Marianas was given priority over the Palaus. However, the target date of 15 June 1944 was not set until 12 March 1944, and Nimitz assigned FORAGER to 5 Fleet (Spruance) on 28 March.

Turner had already begun planning by that date. About a week before 5 Fleet received its formal directive, Turner had concluded that Saipan should be invaded first. Saipan had the best airfields, and its capture would cut off the islands to the south. Turner divided his force into a Northern Force under his personal command for the landings on Saipan and Tinian, and a Southern Force under Conolly for the Guam invasion. The assault elements for Northern Force would be provided by 2 and 4 Marine Divisions under Holland Smith and the assault elements for Southern Force would be provided by 3 Marine Division and 1 Provisional Marine Brigade under Geiger. The floating reserve under Blandy would consist of Ralph Smith's 27 Division, and the invasion would be covered by the fast carrier forces under Mitscher and land-based aircraft under Hoover. Lockwood's submarines would scout well to the west while logistical support would be provided by Calhoun's service force.

American carriers struck the Marianas repeatedly, beginning on 23 February 1944. This was the first good look at the Marianas in over two years, and the raiding aircraft brought back a wealth of photographic intelligence. The raid also destroyed 168 Japanese aircraft and sank 45,000 tons of shipping. Land-base aircraft of 5, 7, and 13 Air Forces, mostly heavy bombers conducting night raids, bombarded Japanese bases in the Carolines throughout March to ensure there would be no Japanese interference with FORAGER from the south. Starting on 18 April, photoreconnaissance aircraft (B-24s) from VD-1, VD-3, and VMD-254 from Guadalcanal began staging through Eniwetok to map the Marianas. These were joined by VD-4 based on Eniwetok itself.

Preinvasion strikes began on 11 June. These came as a rude shock to the Japanese, who had not imagined that the Americans could exploit their successes in the Marshalls and against Truk so quickly, and who in any case believed the Palaus would be the next target. The Americans had also carried out an extensive deception campaign, WEDLOCK, to suggest the next assault would come in the Kuriles. The initial fighter sweeps took place in the afternoon in an attempt to achieve tactical surprise, since previous invasions had begun with dawn raids. 

Landings commenced on 15 June on Saipan, but landings scheduled for the next day on Guam were postponed when the Japanese carrier fleet was spotted by the American submarines. The Battle of the Philippine Sea ended on 20 June with a decisive American victory.  Saipan itself fell on 9 July after a hard-fought campaign.

Landings began on Guam on 21 July and on Tinian on 24 July. The Americans secure Tinian on 1 August and Guam on 10 August.

Once in American hands, the airbases on Saipan were expanded, and new strategic air bases were built on Tinian. These airbases brought the B-29 within range of the Japanese home islands. It was from Tinian that the B-29s carrying the first nuclear weapons were launched against Japan.

References

Gilbert (1989)

Leckie (1962)

Miller (1991)
Morison (1953)

Rottman (2002)

Spector (1985)

Tillman (2005)


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