The fighting started when United States and Australian forces landed on the southwest corner of Morotai, a small island in the Netherlands East Indies (NEI), which the Allies needed as a base to support the liberation of the Philippines later that year.

The island's base facilities were further expanded in 1945 to support the Australian-led Borneo Campaign, and Morotai remained an important logistical hub and command center until the Dutch reestablished their colonial rule in the NEI.

[2] In early 1944, Morotai became an area of importance to the Japanese military when it started developing the neighbouring larger island of Halmahera as a focal point for the defence of the southern approaches to the Philippines.

[5] Allied code breakers detected the Japanese buildup at Halmahera and Morotai's weak defenses, and passed this information on to the relevant planning staff.

[6] In July 1944, General Douglas MacArthur, the commander of the South West Pacific Area, selected Morotai as the location for air bases and naval facilities needed to support the liberation of Mindanao in the Philippines.

This schedule allowed the main body of the United States Pacific Fleet to simultaneously protect both operations from potential Japanese counter-attacks.

[8] As little opposition was expected at Morotai, Allied planners decided to land the invasion force close to the airfield sites on the Doroeba Plain.

[12] The Japanese force was too small and widely dispersed to be able to mount an effective defense, so the 32nd Division ordered it to build dummy camps and use other deceptions in an attempt to trick the Allies into thinking that Morotai was strongly held.

The Tradewind Task Force also included large numbers of construction and other line of communications units whose role was to swiftly develop the island into a major base.

Although the Tradewind Task Force had little information on the invasion beaches or Japanese positions, the Sixth Army did not land any of its reconnaissance patrols on Morotai, as it was feared that these could warn the island's defenders that an attack was imminent.

[24] The Tradewind Task Force embarked onto the invasion convoy at several bases in north-west New Guinea, and conducted landing rehearsals at Aitape and Wakde Island in early September.

[28] While the limited pre-invasion intelligence suggested that Red and White beaches were capable of supporting an amphibious landing, they were in fact highly unsuitable for this purpose.

Many local civilians subsequently provided NICA with intelligence on Japanese dispositions on Morotai and Halmahera and others acted as guides for American patrols.

After dropping two bombs, the TBF Avenger guided USS Richard M. Rowell to the area and the destroyer escort sank Seawolf after five attempts, killing all the submarine's crew.

The PT boats' primary mission was to prevent the Japanese from moving troops from Halmahera to Morotai by establishing a blockade of the 12-mile (19 km)-wide strait between the two islands.

[45] Construction work was accelerated from 18 October after the United States Third Fleet was withdrawn from providing direct support to the planned landing at Leyte.

[47] Although the air base construction required the destruction of villages, the American and Australian airfield engineers were assisted from 1 October by about 350 laborers locally recruited by the NICA detachment.

A jetty for oil tankers and a larger tank farm were completed in early October, and storage facilities continued to be expanded until November, when capacity for 129,000 barrels (20,500 m3) of fuel was available.

Other major construction projects included an extensive road network, a naval installation, 28,000 square feet (2,600 m2) of warehousing, and clearing land for supply dumps and bivouacs.

In an attempt to disrupt the airfield construction program, the Japanese Army commanders on Halmahera sent large numbers of reinforcements to Morotai between late September and November.

[52][1] Allied codebreakers were often able to warn the forces at Morotai of attempts to run the blockade,[6] and PT boats destroyed a large number of the barges the Japanese used to transport troops from Halmahera.

The troops brought to the island suffered from high rates of disease and it proved impossible to bring sufficient supplies through the Allied air and naval blockade.

After landing on the island's west coast, the American regiment moved into Japanese-held territory on 26 December and advanced on the Japanese position from the south-west and north.

The 136th was supported by a battalion of the 130th Infantry Regiment advancing overland from the Doroeba Plain, artillery units stationed on islands off Morotai's coast and one hundred civilian porters.

[56] The 3rd Battalion of the 167th Infantry Regiment also participated in this operation and made a difficult march from Morotai's south coast into the interior to prevent the Japanese from scattering into small groups in the island's mountains.

Both American battalions resumed their attack the next day with the support of a highly effective artillery bombardment, and reached the main Japanese position in the afternoon.

As well as patrolling around Morotai, the boats operated in the eastern NEI to raid Japanese positions and support Australian and Dutch scouting parties.

In May 1945 PT boats and the Australian Z Special Unit rescued the Sultan of Ternate along with his court and harem during an operation codenamed Project Opossum after he was mistreated by the Japanese.

[73] On 9 September 1945, Australian General Thomas Blamey accepted the surrender of the Japanese Second Army at a ceremony held on the I Corps' sports ground at Morotai.

The Australian force responsible for the occupation and military administration of the eastern NEI was headquartered at Morotai until April 1946, when the Dutch colonial government was reestablished.