Admiral King had always advocated using American naval power to attack the Japanese in the central Pacific, but MacArthur had argued for resources to enable him to advance through New Guinea toward the Philippines. If both approaches could be sustained, then they would throw the Japanese off-balance, but it was not until the latter months of 1943 that the US navy began to gather the strength necessary to prosecute an island-hopping campaign in the central Pacific. At the time of Pearl Harbor, the US navy had only three carriers in the Pacific; by late 1943 Nimitz had 10 fast large and medium carriers, seven escort carriers, and a dozen battleships. These formed the key elements of the Fifth Fleet under Vice-Admiral Spruance.
Toward the end of 1943 this force began conducting raids on Japanese island bases, and on November 20, 1943, the US 2nd Marine Division and army units landed on Tarawa and Makin atolls in the Gilbert Islands. Tarawa was a bitter fight; in a savage and bloody five-day battle the Marines lost 1,000 killed, but the Japanese their entire garrison of 5,000. However, Makin was captured relatively easily. American attention now turned to the Marshall Islands with the US Navy’s fast carrier task force raiding the islands in late December 1943 and early January 1944. On January 31, US Marine and army troops landed on Kwajalein Island. Eniwetok fell on February 17, six weeks ahead of schedule. Meanwhile, American carriers under Rear-Admiral Marc Mitscher heavily raided the Japanese naval base at Truk.
With the US navy moving faster than expected in the central Pacific, MacArthur was fearful of being left behind and on February 29, 1944, in a daring raid, his forces seized Los Negros in the Admiralty Islands. All ideas of attacking Rabaul were now abandoned; the huge Japanese garrison was to play little further part in the war. Instead, MacArthur directed a series of landings by American troops along the northern New Guinea coast that isolated 40,000 Japanese forces in the Wewak area. His forces took Aitape and Hollandia on April 23, Wakde on May 17, Biak on May 27, Noemfoor on July 2, and Sansapor on July 30. In three months of island-hopping and fierce fighting, he had advanced over 850 miles (1,400km). With no carriers of his own, and receiving limited carrier support from the Central Pacific, MacArthur’s forces constructed airfields at each landing to provide land-based air support for the next assault.
While MacArthur was advancing, Nimitz was focusing on the Mariana Islands. The key islands were Saipan, Guam, and Tinian, whose airfields were within bombing range of Japan. Realizing the danger, the Commander-in-Chief of the Japanese Combined Fleet (now Admiral Toyoda Soemu) ordered nine carriers and 450 aircraft to gather for a concerted attack on the Americans. Admiral Spruance commanded the invasion, to be covered by Mitscher’s Task Force 58, now with 15 carriers and 1,000 planes. The invasion force included nearly 130,000 troops (only 22,500 fewer than in the opening phase of Operation Overlord at Normandy on June 6, nine days earlier). The invasion force was carried in 535 ships.
Carrier strikes began on June 11, 1944, with troops of the 5th Amphibious Corps under Marine Lieutenant-General Holland Smith landing on Saipan on June 15. Japanese carrier and land-based aircraft attacked the US fleet on June 19, but were totally outclassed by the American aircraft and their more skillful pilots. In the "Great Marianas Turkey Shoot", the Japanese lost 400 aircraft, while the USA lost only 29. During this naval battle three Japanese carriers were sunk by the US aircraft and two by American submarines. Onshore, the Marine and army troops had a savage battle against 32,000 defenders. The Japanese conducted suicide charges, while Japanese civilians leapt to their death from high cliffs as they had been told by their Japanese commanders that the Americans would do terrible things to the civilian populations. By July 9, after 23 blood-drenched days of ferocious fighting, the US Marines had secured Saipan. Total Japanese deaths numbered 30,000. Meanwhile, US Marine and army troops captured Guam and Tinian. The defeat of the Japanese carrier force and the seizure of the Marianas were a devastating blow to the Japanese high command. On July 18, Tojo resigned as Prime Minister and War Minister, succeeding him Lieutenant-General Koiso Kuniaki.