The Battle of North Borneo took place during the Second World War between Allied and Japanese forces. Part of the wider Borneo campaign of the Pacific War, it was fought between 10 June and 15 August 1945 in North Borneo (later known as Sabah). The battle involved a series of amphibious landings by Australian forces on various points on the mainland around Brunei Bay and upon islands situated around the bay. Japanese opposition to the landings was sporadic initially, although as the campaign progressed a number of considerable clashes occurred and both sides suffered significant casualties, although major combat was largely restricted to Labuan and around Beaufort. On the mainland, while Allied conventional operations focused largely on the coastal areas around Brunei Bay, guerrilla forces consisting of Dayak tribesmen and small numbers of Allied personnel from the Services Reconnaissance Department fought an unconventional campaign in the interior. The Allies were successful in seizing control of the region. Nevertheless, many of the strategic gains that possession of North Borneo provided were ultimately negated by the sudden conclusion of the war in August 1945.
Background
Strategic situation and planning
Codenamed Operation Oboe Six, the battle was part of the second phase of the Allied operations to capture the island of Borneo. North Borneo had been occupied by troops from the Imperial Japanese Army since early 1942 following the Japanese invasion of Borneo; prior to this the area had been a British territorial possession. Following its occupation, the area's oil resources had been exploited for the Japanese war effort. The island's population had also been subjected to harsh occupation policies. This had led to a revolt at Jesselton in late 1943, which was suppressed by the Japanese with heavy civilian casualties.
The first stage of the Allied campaign in Borneo had begun in May 1945 when a brigade-sized force had been put ashore on Tarakan, on the north-eastern side of Borneo. The operation in North Borneo was planned by General Douglas MacArthur's South West Pacific Area command. Designed with three phases—preparatory bombardment, forced landings, and an advance—the objective of the Allied operation was to establish "an advanced fleet base" for the British Pacific Fleet in Brunei Bay, which offered the Allies a deep-water port, to enable subsequent naval operations. Further objectives included capturing the vast oil and rubber supplies available in the area and re-establishing British civil administration. It was also intended that Labuan would be secured to control the entrance to Brunei Bay, and would be developed as an airbase. In the planning phase of the operation, the Allied high commands differed in their opinions about the necessity of securing Brunei, with the British Chiefs of Staff Committee believing it would take too long to develop the area for it to be developed for it to be used in operations. They were also concerned that it would divert the British Pacific Fleet from the main theatre of operations off Japan and instead favoured establishing a fleet base in the Philippines. The United States Joint Chiefs of Staff, however, approved the operation believing that it could support future operations in south-east Asia.
In preparation for the landings, commencing in March, the Allied Services Reconnaissance Department (also known as Special Operations Australia) began Operation Agas in North Borneo, and Operation Semut in Sarawak; these were clandestine operations to gather information and organise local Dayak tribesmen to carry out guerrilla operations following the main landings. Ultimately, five Allied parties would be inserted into Borneo as part of Operation Agas, while four were deployed under Semut. Preliminary aerial bombardment of northern Borneo by Australian and US aircraft began on 3 May, before being concentrated on the main landing areas on 5 June. Meanwhile, minesweepers began to clear sea lines of communication for the large Allied naval task force that was assigned to support the operation. This force was designated as Task Force 78.1 and consisted of Australian and US warships, under the command of Rear Admiral Forrest B. Royal. Initially, the Allies planned to launch operations in North Borneo in late May, but shipping shortages delayed moving the assault troops to their staging base on Morotai Island and resulted in the operation being delayed until early June.