The Japanese military knew that holding on to these islands was essential to their control of the shipping routes to Southeast Asia, their retention of the Philippines, and the defense of Japan itself. In consequence, the Marshalls would prove much more difficult to seize than were the Gilberts and Marshalls.
For the U.S. commander of Central Pacific forces, Admiral Chester W. Nimitz, the primary objective in Operation forager was to secure Guam, Saipan, and Tinian. Vice Admiral Raymond Spruance's Fifth Fleet would conduct forager. The main naval component was Vice Admiral Marc Mitscher's Task Force 58 of four fast carrier battle groups. Also under Spruance was Vice Admiral Kelly Turner's V Amphibious Force. Its task was to put ashore the ground force of nearly six Marine and army divisions under Lieutenant General Holland Smith. The forces involved in forager deployed from as far away as Hawaii, 5,000 miles from the Marianas.
Beginning on 23 February 1944, Mitscher's carrier aircraft repeatedly attacked Japanese air bases in the Marianas, virtually wiping out Japanese airpower. On 15 June, the 2nd and 4th Marine Divisions went ashore on Saipan. Despite intense naval bombardment that pounded the island for two days in advance of the invasion, the Japanese defenders held fast. The Marines did not consolidate their beachhead until late that night, and with army reinforcements, they pushed inland over the next two weeks.
The Japanese had anticipated the U.S. invasion, and on its execution, Admiral Toyoda Soemu, commander of the Japanese Combined Fleet, ordered the 1st Mobile Fleet to launch Operation a-go, an effort to destroy the Fifth Fleet. The heavily outnumbered Japanese naval forces were supported by land-based aircraft from the Marianas and nearby islands. To meet this Japanese attack, Spruance delayed the amphibious assault on Guam scheduled for 18 June.
From 19 to 21 June, U.S. and Japanese naval forces met in the Battle of the Philippine Sea, essentially a contest of naval airpower and one of the largest and most decisive battles of the war. Superior numbers and pilot proficiency, as well as radar, carried the day for the Americans. The three-day Battle of the Philippine Sea cost the Japanese 3 large carriers and more than 480 aircraft, in what came to be known as the "great Marianas turkey shoot." U.S. losses came to 130 aircraft, with light damage to the battleship South Dakota. Spruance then ordered Task Force 58 to fall back on the Marianas, to protect the U.S. shipping there. Spruance was criticized at the time and since for his failure to pursue the Japanese fully, but his primary mission was to cover the assault in the Marianas.
Meanwhile, the struggle for Saipan raged. The Japanese commanders on the island, Lieutenant General Saito Yoshitsugu and Vice Admiral Nagumo Chuichi, both committed suicide on 6 July as the remaining Japanese defenders launched wave after wave of suicide charges, the largest of the war; this accelerated the U.S. victory. Thanks to Japan's effective anti-American propaganda, more than 8,000 civilians committed suicide, many by throwing themselves off cliffs on the northernmost end of the island.
Bombardment of Tinian began before Saipan fell on 13 July. The landings on Tinian occurred on 24 July. Marines stormed Tinian's northern coast and encountered little opposition. The situation soon changed, however, when the Japanese defenders regrouped and attacked the Marines in fanatical but futile suicide charges. Tinian was secured by 2 August.
Guam, the most important of the Marianas, had been a U.S. possession for 40 years prior to the war. Actual fighting for Guam began on 21 July, when three American divisions executed near perfect amphibious landings following two weeks of preinvasion bombardment. The ground troops, commanded by Marine Major General Roy Geiger, faced difficult terrain that favored the fanatical Japanese resistance. The Americans secured Guam by 10 August, although mopping-up operations continued in some areas until the end of the war.
The capture of the Marianas accomplished five key objectives of the Pacific Campaign. The "great Marianas turkey shoot" ended the threat from Japanese carrier-based aviation. Additionally, Guam became one of three major western bases for the Pacific Fleet, and the United States established several air bases on these islands from which to launch massive raids against Japan. U.S. forces were now in position to strike at the Philippines and cut off Japan's oil supply from the East Indies. The U.S. victory in the Marianas also prevented any further Japanese resistance to Allied operations in New Guinea. Finally, the loss of Saipan led to the collapse of Tojo Hideki's government and his replacement as premier by Koiso Kuniaki. William P. McEvoy and Spencer C. Tucker
Crowl, Philip A. United States Army in World War II: The War in the Pacific—Campaign in the Marianas. Washington, DC: U.S. Government Printing Office, 1960.; Morison, Samuel Eliot. History of United States Naval Operations in World War II. Vol. 5, The Struggle for Guadalcanal, August 1942–February 1943. Boston: Little, Brown, 1949.
William P. McEvoy and Spencer C. Tucker