Designated collectively as Operation Oboe, a series of amphibious assaults between 1 May and 21 July 1945 were conducted by the Australian I Corps, under Lieutenant-General Leslie Morshead, against Imperial Japanese forces who had been occupying the island since late 1941 – early 1942.

The main Japanese formation on the island was the Thirty-Seventh Army under Lieutenant-General Masao Baba, while the naval garrison was commanded by Vice-Admiral Michiaki Kamada.

The campaign was initially planned to involve six stages, but eventually landings were undertaken at four locations: Tarakan, Labuan, North Borneo and Balikpapan.

[15] Tarakan was also seen to offer a forward airbase to support future operations in the region,[16] while Brunei Bay was intended to be used as naval base.

[23] In the planning phase the commander of the Australian Military Forces, General Thomas Blamey recommended against the landing at Balikpapan, believing that it would serve no strategic purpose.

Blamey was able to frustrate MacArthur's plans for Australian troops to make follow-on landings in Java by convincing Prime Minister John Curtin to withhold the 6th Division.

[26] In late 1944 the Japanese command judged that Australian forces were likely to attack the Brunei area and then capture the west coast of Borneo as part of a campaign aimed at liberating Singapore.

Accordingly, most of the IJA units in north-eastern Borneo were ordered to move overland to the west coast; this required them to undertake gruelling marches over rugged terrain.

[38] Most of the units which had been ordered from north-eastern Borneo to the west coast were still in transit when the Australian landings began and had been greatly weakened by the difficult conditions experienced during their cross-island march.

[42] Using Australian-built MK III folboats, small parties of reconnaissance troops paddled into the Tarakan region to obtain useful information and observe the Djoeta oilfields prior to an invasion.

[44][45] The Australian assault was preceded by a heavy aerial bombardment commencing 12 April, and undertaken by RAAF and US aircraft operating from Morotai Island and the Philippines; these aircraft interdicted Japanese shipping, attacked airfields, reduced obstacles around the landing beaches and suppressed artillery and defensive positions;[46] in addition, a preliminary landing was undertaken on Sadau Island by a group of commandos and an artillery battery.

[16] Naval assets assigned to provide fire support during the operation include three cruisers, seven destroyers and several landing craft fitted with rockets and mortars.

Japanese resistance grew, and the advance was hindered by large numbers of mines and booby traps, which had to be cleared by engineers and pioneers; however, finally, on 5 May the airfield was captured by the Australians.

[56] While the 24th Brigade rapidly captured Labuan's airfield and town, it encountered sustained resistance from the Japanese garrison which was located in a well-defended stronghold.

[59] A week after the initial landing on Labuan, the Australians followed up with attacks on Japanese positions around Weston on the north-eastern part of Brunei Bay.

[60][61] The Australians then pushed inland along the single track railway that ran from Weston towards the junction at Beaufort, 23 kilometres (14 mi) north-east of Brunei Bay,[62][63] and then on to Jesselton.

[66] The operations in North Borneo also involved a substantial civil affairs effort to assist the liberated civilian population; this was the largest such task undertaken by Australian forces during World War II.

The 9th Division was heavily involved in providing aid to civilians and rebuilding houses and infrastructure which had been destroyed by the pre-invasion bombardments and subsequent fighting.

The inhabitants of Sarawak's interior, who were known as Dayaks, enthusiastically joined the guerrilla groups and SOA personnel led small private armies.

200 Flight RAAF and the Royal Australian Navy's Snake-class junks played important roles in this campaign by inserting SOA personnel and supplies.

[71] The guerilla forces launched attacks to gain control of the interior of Sarawak while the 9th Division focused on the coastal area, the oilfields, plantations and the ports in North Borneo.

[72] The guerillas operated from patrol bases around Balai, Ridan and Marudi, as well as in the mountains, along several key waterways including the Pandaruan and Limbang Rivers, and along the railway that ran between Beaufort and Tenom.

[73] The RAAF flew air strikes to support the lightly armed guerillas, who at times had to evade better-armed Japanese units.

Due to the unavailability of the Tarakan airfield, air support for the operation was provided by RAAF and US units based in the southern Philippines.

[76] A deception plan was enacted to draw Japanese attention towards Manggar, with the Australians undertaking pre-landing operations and spreading disinformation.

[82] Along the coast, the 21st Brigade crossed the Batakan Ketjil River, where it was held up on 3 July by strong Japanese resistance, which was overcome with naval gunfire support.

Meanwhile, the airfields were secured by 9 July, but Japanese resistance was strong, utilising delaying tactics including booby traps, mines, sniping and small-scale raids.

[85] Moving inland along a north-east road dubbed the "Milford Highway" by the Australians, the 25th Brigade advanced to contact with the Japanese rearguard, which was reduced with artillery support and then outflanked, triggering a withdrawal to a secondary position 3 miles (4.8 km) back on 9 July.

[92] Despite the success of the Allied landings, because of the strategic situation, the Borneo campaign was criticised in Australia at the time and in subsequent years, as pointless or a "waste" of the lives of soldiers,[93] especially following the first operation in Tarakan.

According to Hastings, "Any rational strategic judgment would have left them to their own devices screened by token allied forces until their nation's defeat enforced their surrender".