The 'Campaign for North Borneo' was fought between Allied (primarily Australian) and Japanese forces as part of the wider Borneo campaign of the Pacific War (10 June/15 August 1945).

The campaign involved a series of amphibious landings by Australian forces at a number of points on the mainland around Brunei Bay and islands situated around the bay. Japanese opposition to the landings was at first only sporadic, but as the campaign continued there occurred a number of considerable clashes in which each side suffered significant casualties, although major combat was restricted largely to Labuan island and the area round Beaufort. On the mainland, while Allied conventional operations were focused largely on the coastal areas round Brunei Bay, guerrilla forces (Dayak tribesmen and small numbers of Allied personnel of the Australian Services Reconnaissance Department) fought an unconventional campaign in the interior. The Allies were successful in seizing control of the region, but many of the strategic gains that possession of North Borneo provided were ultimately proved pointless by the sudden conclusion of the war against Japan on 15 August 1945.

Codenamed 'Oboe VI', the battle was part of the second phase of the Allied operations to capture the island of Borneo. A British possession, North Borneo had been occupied by Japanese troops since a time early in 1942 following the Japanese 'B' (ii) invasion of Borneo. After its occupation, the area’s oil resources had been exploited for the Japanese war effort. Some the island’s population was subjected to the extreme harshness of the occupation. This had led to a revolt in Jesselton late in 1943, and this had been suppressed by the Japanese with heavy civilian casualties.

The first stage of the Allied campaign to regain Borneo, which had both British and Dutch territories, had begun in May 1945 when a brigade-sized force had been put ashore in 'Oboe I' on Tarakan, an island just off the north-eastern coast of Borneo. The operation in North Borneo was planned by General Douglas MacArthur’s South-West Pacific Area command within its 'Princeton' preliminary and 'Montclair' definitive schemes for the recapture of the Philippine islands group, Borneo and the Netherlands East Indies. Designed with three phases (preparatory bombardment, opposed landings and advances), the objective of the operation was to establish an advanced fleet base for Admiral Sir Bruce Fraser’s British Pacific Fleet in Brunei Bay, which offered the Allies a deep-water port for the furtherance of subsequent naval operations. Other objectives included the seizure of the great oil and rubber supplies available in the area, and the re-establishment of British civil administration. It was also intended that Labuan island would be secured to control the entrance to Brunei Bay, and would be developed as an air base. In the planning phase of the operation, the Allied high commands differed in their opinions about the necessity of securing Brunei, the British Chiefs-of-Staff Committee believing it would take too long to develop the area for it to be useful for other operations. The British were also concerned that it would divert the British Pacific Fleet from the main theatre of operations off Japan, and the British therefore favoured the establishment of a fleet base in the Philippine islands group. The US Joint Chiefs-of-Staff, on the other hand, approved the operation in the belief that it could support future operations in South-East Asia.

In preparation for the landings, from March 1945, the Allied Services Reconnaissance Department (also known as Special Operations Australia) began its 'Agas' undertakings in North Borneo and its 'Semut' undertakings 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 in 'Agas' and four in 'Semut'. Preliminary air 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 Task Force 78.1 comprised Australian and US warships under the command of Rear Admiral Forrest B. Royal. Initially, the Allies planned to launch operations in North Borneo late in May, but shipping shortages delayed the movement of the necessary assault troops to their staging base on Morotai island, so the operation was postponed until a time early in June.

A total of between 29,000 and 30,000 Allied troops was committed for the recapture of North Borneo, the majority of the ground forces being provided by Major General G. F. Wootten’s Australian 9^th Division, which comprised the 20^th, 24^th and 26^th Brigade Groups. At the time of 'Oboe VI', however, the 26^th Brigade Group was engaged at Tarakan in 'Oboe I' after being detached in May 1945, so only two brigade groups were available for 'Oboe VI'. An element of the all-volunteer 2^nd Australian Imperial Force, the 9^th Division was a veteran formation, having previously served in North Africa, the Middle East and New Guinea. Before the Borneo campaign, the division had been rested and reorganised on the Atherton Tablelands in Queensland. The division had experienced a high turnover of personnel following its service in the Huon peninsula campaign in New Guinea, as soldiers were medically discharged or transferred to other units. In addition to the Australian ground troops, naval support was provided by the US Navy and Royal Australian Navy, and air support by Major General Paul B. Wurtsmith’s US 13^th Army Air Force, the US Marine Corps, and elements of the Royal Australian Air Force’s 1^st Tactical Air Force. Two US Army units, the 727^th Amphibian Tractor Battalion, whose men manned the LVTs, and the 593^rd Engineer Boat and Shore Regiment’s Boat Battalion, were also attached to the Australians.

Meanwhile, Allied intelligence estimated that there were about 31,000 Japanese troops on Borneo, about 8,800 of these in North Borneo. Lieutenant General Maseo Baba’s 37^th Army, headquartered in Jesselton, was responsible for the defence of the area. The main Japanese units in the vicinity included elements of Major General Taijiro Adachi’s 56^th Independent Mixed Brigade, comprising six battalions (the 366^th Battalion to the 371^st Battalion) as well as one independent battalion. The brigade had been raised in Japan during the second half of 1944 and had reached Borneo late in that same year as the area’s garrison troops were reorganised for defence against the possibility of future Allied landings. By the middle of 1945, the brigade had been heavily depleted by its overland movement from the north-eastern part of Borneo even before the start of the Allied landings and was thus at about half strength; moreover, its troops were largely inexperienced, lightly equipped and suffering from poor morale. Japanese air power in the region had been heavily depleted and, except in Java and Sumatra, was ineffective, although there were small numbers of aircraft at Keningau and Kuching.

Two main landings were undertaken by the Australians in North Borneo on 10 June. After concentrating in May at Morotai island, where landing rehearsals were undertaken, the assault force of 85 predominantly US ships departed early in June, preceded by minesweepers and survey vessels, as well as the naval attack group. The first landing was made when men of two battalions of Brigadier S. H. W. C. Porter’s 24^th Brigade Group (the 2/28^th and 2/43^rd Battalions) landed on Labuan island together with one squadron of Matilda II infantry tanks of the 2/9^th Armoured Regiment. The 24^th Brigade Group’s third battalion, the 2/32^nd Battalion, was kept in divisional reserve for the initial landing. The attack was preceded by a heavy naval bombardment from cruisers, mortar and rocket ships, and attacks by eight Consolidated B-24 Liberator four-engined heavy bomber squadrons, which dropped anti-personnel bombs to target Japanese troops around the planned beach-heads. With this support, the main Allied landings were largely unopposed as the Japanese defenders had withdrawn from the beaches on the peninsula, and nearby Muara island had been abandoned completely. At Labuan, the Australian troops came ashore near Victoria and, supported by a heavy artillery and naval gunfire, the two battalions drove toward the airfield. Light opposition was overcome and the town and airfield were secured late on the first day, after minor clashes with Japanese outposts and troops fighting among the aircraft dispersal bays. Meanwhile, the 2/11^th Commando Squadron provided flank support to the west.

Despite the initial progress, the fighting on Labuan intensified during this time as the Japanese defenders feel back inland to a heavily fortified position known as 'The Pocket' and attempted to hold the Australians along the dense jungle ridges and thick swamps. The 2/12^th Commando Squadron was brought ashore from divisional reserve on 12 June and given the task of clearing the outlying areas of resistance which had been bypassed during the initial advance. By 14 June, the Australians had secured the island, apart from those Japanese in 'The Pocket'. Despite considerable artillery and armoured support, a company-level attack by the 2/28^th Battalion was turned back on 14 June, and as a result further preparatory fire was used to soften the Japanese defences.

At this stage of the war, Australian commanders were under strict instruction to limit their casualties by avoiding unnecessary risks and making use of fire support where possible to reduce Japanese defences before any land assault. Starting on 17 June, an intense three-day naval and air bombardment was laid down in an effort to reduce the Japanese defences. Meanwhile, 100 Japanese attacked the Australian brigade’s maintenance area and the airfield before the Australians launched a renewed attack on 21 June. At this time, two infantry companies of the 2/28^th Battalion assaulted the Japanese position. Supported by indirect fire support from sea and air, and direct fire support from tanks and flamethrowers, the Australians overwhelmed the Japanese defenders and cleared the remaining resistance from Labuan. After the battle 180 Japanese dead were counted, bringing the total killed during the fighting on Labuan to 389. Against this the Australians lost 34 men killed and 93 wounded.

The second Allied landing on 10 June consisted of two battalions of Brigadier W. J. V. Windeyer’s 20^th Brigade Group (2/15^th and 2/17^th Battalions), which landed on Muara island and on the mainland peninsula to the north of Brooketon, supported by a second squadron of Matilda II infantry tanks of the 2/9^th Armoured Regiment. The 20^th Brigade Group’s third battalion, the 2/13^th Battalion, was held back in brigade reserve. Meanwhile, in the interior, Dayak tribesmen supported by Allied operatives began their guerrilla campaign on 9 June. Lightly armed, and with only limited training, these guerrillas harassed the withdrawing Japanese while avoiding decisive engagement. In this role, they met with limited success, but were in some cases forced to withdraw in the face of heavy opposition. The troops which had landed near Brooketon on the mainland advanced on Brunei, which was captured on 13 June by the 2/17^th Battalion after a number of minor section- and platoon-level actions over several days. The 2/15^th Battalion, which had earlier secured Muara island, took Limbang on 18 June, advancing by landing craft up the river in the south-west of Brunei Bay. The 20^th Brigade Group’s two initial battalions were now joined by the 2/13^th Battalion, which had conducted an unopposed landing at Lutong on 20 June, supported by Supermarine Spitfire and Curtiss Kittyhawk single-engined fighter-bombers operating from Labuan, before continuing their advance down the south-western coast and then overland, passing through Miri and Seria on their way towards Kuching.

At Seria the Australians found the 37 oil wells ablaze, having been deliberately lit by the Japanese defenders as they withdrew, and engineers from the 2/3^rd Field Company were called forward to extinguish the fires, a task which required more than three months to complete. Kuala Belait was reached on 24 June. Having secured its objectives, the 20^th Brigade Group then began patrol operations, using landing craft to move quickly along the various rivers and streams punctuating the coast. The initial priority of the Japanese troops on the mainland was to withdraw inland. As a result, only minor clashes occurred, against Japanese rearguards, which were generally poorly equipped and inexperienced. Resistance and aggression among these rearguard elements stiffened as the Australians moved beyond Miri. The guerrilla forces in the interior generally carried out their operations separately from the conventional forces focused mainly in the coastal areas. However, some co-ordinated action was achieved during the campaign. During July, 'Semut' guerrillas captured Marudi, on the Barem river, as part of efforts to disrupt the Japanese withdrawal from Miri. A strong Japanese counterattack retook the village from the lightly armed 'Semut' operatives, after which the guerrillas linked with Australian conventional infantry of the 2/17^th Battalion to capture it once again on 15 July. During the course of its involvement in the campaign, the 20^th Brigade Group suffered only relatively light casualties totalling 40 men. Throughout the period from late in June and into August, RAAF aircraft including de Havilland Mosquito twin-engined light bomber and Bristol Beaufighter twin-engined heavy fighter-bombers attacked Japanese targets throughout North Borneo: the targets included barges, shipping, barracks and airfields: one 800-ton vessel was sunk near the Tabuan river and several Japanese aircraft were destroyed on the ground. Commonwealth Wirraway single-engined light attack aircraft were also used to provide tactical reconnaissance, and other fighters flew close air support sorties.

Another landing was made by Allied forces on 16 June on the mainland at Weston, in the north-eastern part of Brunei Bay. Previously held back as the divisional reserve, the 2/32^nd Battalion forced its way ashore near Padas Bay and, after it had taken Weston, sent patrols to Beaufort, some 14 miles (23 km) inland. For lack of roads and the indefensible nature of the railway track to the town, it was decided to advance along the Klias river, while a secondary force moved along the Padas river. As a part of this phase of the operation, minor landings were made at Mempakul on 19 June and at Sabang on 23 June by elements of the 2/43^rd Battalion and the 2/11^th Commando Squadron. Kibidang was captured the same day by the 2/43^rd Battalion, while the 2/32^nd Battalion advanced farther along the Padas river and the two battalions then linked. After this, reinforcements in the form of two companies of the 2/28^th Battalion were transferred from Labuan island to assume responsibility for rear-area security while plans were made for the main attack on Beaufort.

It was the Allied estimation that Beaufort, which lay on the main Japanese avenue of withdrawal, was held by between 800 and 1,000 Japanese troops seeking to keep key egress routes open. On 27 June, the Australians attacked the town. The 2/43^rd Battalion was assigned the task of the main assault, while the 2/32^nd Battalion was tasked with flank protection. Despite being hamstrung by torrential rain and unforgiving terrain, the 2/32^nd Battalion secured the Padas river’s southern bank, while one company of the 2/43^rd was sent to take the town and another marched to the flanks, to take up ambush positions along the route along which the Japanese were expected to withdraw. The 2/28^th Battalion secured the lines of communication to the north of the river. The Japanese defenders' resistance was not co-ordinated and as a result the Australians had secured their objectives by the fall of night. Throughout the night, however, the Japanese launched six counterattacks which eventually became hand-to-hand combat. During the course of these actions, one company became isolated and during the morning of the following day, 28 June, another company was sent to aid it by attacking the Japanese force from the rear. Fighting its way through numerous Japanese positions throughout the afternoon, the company reached its objective early in the evening and launched its assault, killing at least 100 Japanese. By 29 June, the Japanese had started to withdraw from Beaufort in small groups.

Elsewhere, on 1 July, Major General E. J. Milford’s Australian 7^th Division carried out the final stage of the Allied operation to secure Borneo, landing at Balikpapan, on the south-eastern coast in 'Oboe II'. In North Borneo, Allied forces observed a brief pause while reinforcements arrived. The 2/3^rd Anti-tank Regiment, being used in the infantry rather than the anti-tank role, arrived in Weston on 3 July and relieved the 2/28^th Battalion, which then moved to Beaufort. On 6 July the Australian advance was resumed and, as a result of the current strategic situation, it was decided to undertake only a slow and cautious advance using indirect fire to limit Australian casualties. By 12 July the 2/32^nd Battalion had occupied Papar, and from there patrols were despatched to the north and along the banks of the river as offensive operations came to an end.

After the capture of Papar, the Australians ceased offensive actions on Borneo and the situation remained largely static until a ceasefire came into effect on 15 August: on 6 and 9 August, two atomic bombs had been dropped on Hiroshima and Nagasaki, and on 15 August Emperor Hirohito effectively announced an end to hostilities with his 'Imperial rescript'. Japan’s formal surrender was signed on 2 September on the US battleship Missouri in Tokyo Bay. As a result of the ceasefire, the planned Allied 'Olympic' invasion of Japan was no longer required and the strategic gains provided by the capture of North Borneo were arguably negated. This included the development of Brunei Bay as a naval base, which ultimately never occurred. To a certain degree, this has led to claims in Australia that the 'Oboe' operations, as well as the campaigns in the Aitape and Wewak region of New Guinea and on Bougainville and New Britain islands, had been unnecessary and had therefore resulted in needless casualties. Throughout the course of the fighting in North Borneo, the Australians lost 114 men killed or died of wounds while another 221 men were wounded. Against this, the Japanese lost at least 1,234 men, while 130 had been taken prisoner. It is estimated that another 1,800 Japanese had been killed by the guerrilla forces operating in the interior; many of these were Japanese troops who were withdrawing inland following the conventional forces' landings on the coast, and these were ambushed by guerrillas or attacked by Allied warplanes directed by these forces. These forces had also occupied large areas in Sarawak and the southern parts of North Borneo by the end of hostilities.

After the fighting was over, the Australians began the task of re-establishing the British civil administration, rebuilding the infrastructure that had been damaged and providing for the civilians displaced in the fighting. This emerged as a significant undertaking, with the Australian 9^th Division working to establish hospitals, dispensaries and schools. Sanitation and drainage had not been provided by the Japanese, and the local population was suffering from disease and was malnourished. Infrastructure was re-built by Australian engineers, while the medical teams of the Australian 9^th Division provided medical aid directly to locals. The 82-mile (132-km) North Borneo railway was also re-established. Houses that had been destroyed in pre-invasion bombardment and later fighting were also rebuilt. Following the ceasefire, there were still a large number of Japanese troops in North Borneo: by October 1945 it was estimated that there were still more than 21,000 Japanese soldiers and civilians scattered through North Borneo, and the Australian 9^th Division was made responsible for organising the surrender of these people, and also for provisioning and protecting them. The division was also tasked with the liberation of Allied civilian internees and prisoners of war held at Batu Lintang camp in Kuching, Sarawak, and with disarming the 'Agas' and 'Semut' guerrillas.

As civil administration was slowly restored, in October 1945, the Australian demobilisation process began. Initially, this process was slow as there were few troops able to relieve the Australian forces in Borneo and as such only long-service personnel were released for return to Australia. The Australian 9^th Division remained in North Borneo performing garrison duties until January 1946, when it was relieved by the Indian 32^nd Brigade and subsequently disbanded. For the majority of the Australian 9^th Division’s personnel, a return to civilian life followed though, as part of Australia’s contribution to the occupation of Japan, a number of the Australian 9^th Division’s men were transferred to the 67^th Battalion which was being formed as part of Brigadier R. H. Nimmo’s Australian 34^th Brigade.