Although the strong enemy island bases in the eastern Marshalls were bypassed, the air forces maintained continual attacks on them throughout the year. Conquest of the western Marshalls provided air bases and a new forward fleet base in the Pacific.
The Mariana Islands, the next objective in the Central Pacific, differ from the coral atolls of the Marshalls and Gilberts. The individual islands are much larger and the distinguishing terrain features are precipitous coast lines, high hills, and deep ravines. Plans were made, ships and supplies collected, and the troops given special training for the invasion; meanwhile Japanese air and ground reinforcements poured into the Central Pacific.
An intense air offensive against enemy installations in the Marianas began on 11 June 1944 and a naval bombardment of Saipan began on the 13th, two days before the landings on the 15th. Opposition was heavy at first, but by the 25th U.S. troops, supported by tanks, heavy artillery, renewed naval gunfire, and aerial bombardment, drove the enemy from the high ground on the central part of the island. Again advances were slow and difficult with heavy troop losses. On 9 July the mission was completed, except for mopping-up operations which continued for nearly two months.
On the morning of 24 July an attack was made on Tinian, supported by artillery on Saipan. Enemy resistance, slight for first two days, increased when high ground was reached in the central part of the island. The entire island was overrun by 1 August.
Meanwhile, Guam had been invaded on 21 July by U.S. forces in two separate landings. This invasion was preceded by a thirteen-day aerial and naval softening-up process. The two beachheads were joined after three days of fighting. The troops, greatly hampered by heavy undergrowth, concentrated on the high ground in the northern part of the island and, except for resistance from small groups of scattered Japanese, were in command of the island by 10 August.
A force of nearly 800 ships from the Guadalcanal area sailed for the Palau Islands, the next hop in the Central Pacific. Marines landed on Peleliu Island on 15 September while Army units landed on Angaur on the 17th. These were the two southernmost islands of the Palau group. Opposition on Angaur was relatively light. Much stiffer resistance was met on Peleliu, which contained the site of the major Japanese airfield on the islands. The troops succeeded, by 12 October, in pushing the enemy into a small area in the central hills of Peleliu, but many more weeks were spent destroying the remaining opposition.
During the fighting in the southern Palaus, Ulithi Atoll in the western Carolines was taken to secure a naval anchorage in the western Pacific. Air attack against bypassed islands was maintained. Meanwhile, huge air bases were being developed in the Marianas for use by B-29 bombers. On 24 November B-29’s operating from Saipan made the first of a series of attacks on Tokyo.
Concurrent with the operations in the Marshalls, Marianas, Palaus, and Carolines, forces of the Southwest Pacific Area moved swiftly along the northern coast of New Guinea, jumped to Vogelkop Peninsula, and then to Morotai and on into the Philippines. The first amphibious advance of 1944 in this area was made on 2 January at Saidor, to capture the airport there. The next major advance was begun early on the morning of 29 February when a landing was effected on Los Negros in the Admiralty Islands. The Japanese sent reinforcements from Manus Island, separated from Los Negros by only 100 yards of water. Except for isolated groups of enemy troops, Los Negros was cleared on the 23d and Momote airfield, on the east coast, was ready for operation. Manus Island was invaded on 15 March, after the seizure of a few smaller islands, and an airfield there was captured the next day. At the end of April most of the enemy had been cleared from the Admiralties.
In New Britain the beachheads established in 1943 were expanded. On 6 March another landing took place on Willaumez Peninsula on the north coast. This operation, together with the establishment of airfields in the Admiralties and the occupation of Green and Emirau Islands, completed the encirclement and neutralization of Rabaul, the once powerful Japanese base. On 26 November U.S. units left New Britain, the enemy being contained on the Gazelle Peninsula by the Australians.
In New Guinea, after the Saidor operation, the enemy organized his defenses in the coastal area between Wewak and Madang. Surprise landings by U.S. troops were made at Aitape and Hollandia, both west of Wewak, on 22 April. Within five days the airfields at Hollandia and Aitape were in Allied possession. In July 1944 the Japanese Army, which had moved up the coast from Wewak, attacked the Allied perimeter at Aitape. Within a month the Japanese had been thrown back toward Wewak. At the end of the year Australian troops, which had begun relieving U.S. forces at Aitape in October, started a drive on Wewak from the west. While the enemy was bottled up in this area, the Allies continued to leap-frog up the New Guinea coast.