'Oboe VI' was the Australian seizure of Brunei Bay on the north-western corner of Japanese-occupied Borneo as the final Australian contribution to the overall 'Montclair' plan (10 June/1 July 1945).

In 1941, Brunei was a 2,226-sq mile (5765-km²) British protectorate with Brunei Town (Bandar Seri Begawan) as its main city, an important oilfield at Seria, a good export trade in rubber, and a very large sheltered anchorage in Brunei Bay, which separates the two parts of the protectorate. The population in 1941 was about 39,000 persons.

Brunei fell to elements of the Japanese 142^nd Regiment and the 2^nd Yokosuka Special Naval Landing Force on 15/16 December 1941.

Brunei was the most important oil-producing region of Borneo, as indicated by the strength of the Japanese garrison, which was under the overall control of Lieutenant General Masao Baba’s 37^th Army, now headquartered at Melalap due inland from the northern end of Brunei Bay. This army comprised Major General Taijiro Akashi’s 56^th Independent Mixed Brigade (371^st Battalion on Labuan island, 366^th Battalion and 367^th Battalion in Brunei, and 368^th Battalion at Beaufort), the 25^th Independent Mixed Regiment, and detachments of the Japanese navy’s 22^nd Special Base Force and 2^nd Guard Force.

At the start of 'Oboe VI', elements of the 56^th Independent Mixed Brigade were deployed around Brunei Bay and inland just to the north-east of this (the latter concentration as a mobile reserve to react to landings either in Brunei Bay or along the upper part of the north-west coast between Jesselton and Cape Sempang Mangayan. Other elements of the brigade and the 25^th Independent Mixed Regiment were deployed in that sector. These units totalled almost 5,000 men as well as some 650 more on Labuan island and 1,550 others around Seria and Miri some 50 to 70 miles (80 to 115 km) to the south-west of Brunei Bay. Still other elements of the brigade, to a strength of some 1,500 men at Tawau in the St Lucia Bay area, were positioned on the lower south-east coast of North Borneo in the vicinity of the border with Dutch Borneo and to the north of Tarakan island.

Much of the strength at Tawau had been ordered to move overland in March to strengthen the units around Brunei Bay. This was a task of very great difficulty, and the movement cost the Japanese many lives and left the survivors diseased and exhausted, and in possession of only light weapons and limited supplies.

As planned, 'Oboe VI' was a component of the second phase of the Allied undertakings to recapture Borneo, a British colonial possession. North Borneo had been occupied by troops of the Imperial Japanese army since a time early i 1942 following the Japanese 'B' and 'Otsu' invasion of Borneo and subsequent land operations. After 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 late in 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, in 'Oboe I'. The later 'Oboe VI' in North Borneo was planned by General Douglas MacArthur’s South-West Pacific Area command. Designed as undertaking in three phases (preparatory bombardment, amphibious landings and land advance) the objective of the Allied 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, to enable subsequent naval operations. Further objectives included the seizure of the large oil and rubber supplies of the area, and the re-establishment of British civil administration. It was also planned that Labuan island was to be seized as it controlled the entrance to Brunei Bay, and then 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, with the British Chiefs-of-Staff Committee believing it would take too long to develop the area 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 Philippine islands group. The US Joint Chiefs-of-Staff approved the operation, however, in the belief that it could support future operations in South-East Asia.

In preparation for the landings, scheduled to start in March, the Allied Services Reconnaissance Department (also known as Special Operations Australia) began a pair of clandestine operations ('Agas' in North Borneo and 'Semut' in Sarawak) to gather intelligence and to organise local Dayak tribesmen for the task of carrying out guerrilla operations following the main landings. Ultimately, five Allied parties were be inserted into Borneo as part of 'Agas' and four to be deployed in 'Semut'.

The 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 Rear Admiral Forrest B. Royal’s Task Force 78.1 of Australian and US warships. The Allies initially planned to launch operations in North Borneo during a period late in May, but shipping shortages delayed moving the assault troops to their staging base on Morotai island led to the postponement of 'Oboe VI' to a time early in June.

Assembled on Morotai island, Major General George F. Wootten’s 9^th Division (less its 26^th Brigade that was involved in 'Oboe I' on Tarakan island) was assigned the mission of taking this large area. The division was therefore considerably reinforced to a strength of almost 31,000 men with support, service and base personnel supplied by Lieutenant General Sir Leslie Morshead’s I Corps.

The 'Oboe VI' invasion plan was complex, and called for a pair of main landings separated by 20 miles (32 km), one at Labuan and the other on the spit on the south-western side of the bay, as well as a third but smaller landing on Muara island near the location of the second landing.

Like the others in Borneo, the assault area had previously been hemmed with substantial minefields to prevent the movement of Japanese tankers. A large-scale minesweeping effort preceded the landings, starting on 22 April 1945 and continuing to 29 April. After a pause, a second US minesweeper force of five fleet minesweepers and 12 motor minesweepers continued to clear paths through the minefields. This second phase lasted from 7 to 9 June, and in the process, the minesweeper Salute hit a mine and later sank on 8 June, but by 12 June some 102 mines had been cleared.

Cover for the minesweepers and shelling of the assault areas was the responsibility of Rear Admiral Russell S. Berkey’s Task Group 74.3 (Cruiser Covering Force) comprising the US light cruisers Phoenix, Nashville, Boise (carrying General Douglas MacArthur) and the Australian light cruiser Hobart, and the US destroyers Killen, Albert W. Grant, Conner, Charette, Bell and Burns, and the Australian destroyer Arunta. The gunfire bombardment of the Japanese shore positions was continued on 8/9 June.

After completing extensive rehearsals at Morotai island, the 9^th Division was transported by the ships of TG78.1 (Brunei Attack Group) with the headquarters ships Rocky Mount and US Coast Guard cutter Spencer of Vice Admiral Thomas C. Kinkaid’s US 7^th Fleet to the north-west through the Sulu islands archipelago of the Philippine islands group and past the south-western end of Palawan island, and arrived off Brunei Bay on the morning of 10 June. The troops completed the 1,100-mile (1770-km) journey aboard landing ships of TG78.1 rather than troop transports under crowded and generally uncomfortable conditions.

The force which transported and supported the 29,361 troops of the reinforced division was Vice Admiral Daniel E. Barbey’s Naval Attack Force, a combined US and Australian force including Rear Admiral Forrest B. Royal’s 6^th Amphibious Group (Australian infantry landing craft Westralia, Manoora and Kanimbla, one US dock landing ship, two attack cargo ships, nine troop-carrying destroyer conversions, 35 tank landing ships, 21 medium landing ships and 55 infantry landing craft; their escort comprised the US destroyers Robinson, Saufley, Waller, Philip and Frazier (Destroyer Squadron 22) and Bancroft, Bailey, Edwards, Caldwell and McCalla (Destroyer Squadron 14), US destroyer escorts Douglas A. Munro, Charles E. Brannon, Albert T. Harris, Dufilho, Jobb and Day, Australian frigates Barcoo, Hawkesbury and Lachlan, and 12 motor torpedo boats. Support was provided by TG74.3.

Australian and US tactical aircraft undertook an intense suppression effort against the Japanese on Labuan island at Brooketon between 5 and 9 June, and before the landings US and Australian warships (four cruisers and seven destroyers) saturated the assault area with naval gunfire. Brigadier Selwyn H. W. C. Porter’s 24^th Brigade Group then landed on Labuan island and Brigadier W. J. Victor Windeyer’s 20^th Brigade Group at Brooketon.

In overall terms, the 24^th Brigade Group was soon able to secure the airstrip on Labuan island, though the Japanese maintained an organised defence of the western end of the island until 21 June; the 24^th Brigade Group then landed two battalions on the eastern side of Brunei Bay to take Beaufort on 28 June; and meanwhile the 20^th Brigade Group had taken Brunei Town on 15 June before advancing to the south-west along the coast to take Seria on 21 June, while another amphibious landing gave the brigade the towns of Lutong and Miri just across the border in Sarawak.

The 20^th Brigade had landed at two points on the western side of Brunei Bay without opposition. One battalion landed on Green beach at Brunei Bluff defining the western side of the bay’s entrance while another battalion landed on White beach on Muara island just to the south of Brunei Bluff. There were no Japanese troops on the island, and the bay’s entrance was secured. A company from that battalion was sent up the Brunei river at the western side of the bay, followed by the rest of the battalion.

Labuan island in the bay’s entrance and to the east of Brunei Bluff measures about 12 miles (19 km) north-east to south-west along its north-western shore and some 6 miles (10 km) across its centre, where a portion of the island extends out to the south-east. At the end of this protrusion is Victoria Harbour and the main town of Labuan. The northern end of the harbour and the western side of the peninsula on the harbour’s western side are covered with mangrove swamps, and the island is mostly low and flat with a few hillocks and largely forested. Less than 1.5 miles (2.4 km) to the north of the town is Labuan airfield. Roads run north and west out of the town to allow access to those sides of the island. On the centre of the north-west coast was Timbalai airfield. Only a few trails served the southern portion of the island, which was largely undeveloped.

The 24^th Brigade landed two battalions on Labuan island’s south-eastern protuberance and the northern side of Victoria Harbour on Brown beach at 09.15. The brigade’s battalions at first met no resistance, moved inland and then began to encounter resistance in the area to the north of the town. On 12 June the Australians located a Japanese concentration in 'The Pocket', a stronghold area measuring 600 by 1,000 yards (550 by 915 m), on the northern edge of the mangroves at the harbour’s head and to the west of Labuan airfield. The Australians completed the destruction of 'The Pocket' only on 21 June, and the battalions then moved to the north and west to clear the rest of the island. At least 400 Japanese were killed on Labuan, half of them in 'The Pocket'.

On 17 June a third battalion of the 24^th Brigade landed on the eastern side of the bay, and took Weston and then Lingkungan as it pressed inland. On 19 June another battalion landed near the bay’s north-eastern shoulder and, in concert with the battalion to the south, advanced to the east in the direction of Beaufort, which was taken on 27 June.

The 24^th Brigade continued to advance up the coast into mid-July, but did not secure Jesselton, British North Borneo’s capital, until 28 September.

In the meantime, on the south-western side of Brunei Bay, the 20^th Brigade conducted other small-scale landings by a battalion landing team at 09.15 on 10 June to clear the southern shore and, and then moved to the south-west along the coast toward Seria, which was secured on 22 June. One battalion was landed farther to the south-west in the area of Miri and Lutong, some 70 miles (115 km) from the bay, on 20 June. The area had been abandoned by the Japanese, but there were some small pockets of resistance along the coast. The Japanese had fired the 37 oil wells at Seria as they withdrew, but the well-head fires were soon extinguished by engineers. The oilfields at Miri and other oil production facilities were captured intact.

The two Australian elements soon joined. On 27 June the Japanese positions inland were found and attacked with the aid of Dyak guerrillas, and by this time the Japanese were pulling back inland along the whole of this stretch of the North Borneo coast to the north-east or along the north-western coast toward Kuching near Borneo’s western tip, where an untouched Japanese garrison was located. The Australians conducted aggressive patrols to hurry them, but there was no intent to pursue and in the process suffer Australian casualties to no good purpose. The Japanese moving to the north-east gathered along the north-east coast near Sandakan to make a final stand, but were not attacked.

The 9^th Division’s main objective, to secure the coast and key facilities, had been accomplished, and the formation now controlled a 5,000-sq mile (12950-km²) area requiring only limited mopping up and security patrols.

By 12 June 1945 the US minesweeping force had swept a total of 102 mines, and then relocated to the Miri area, where they swept another 338 mines.

After their capture of Papar, the Australian forces ceased offensive actions on Borneo, whereupon the situation remained largely static until a ceasefire came into effect in mid-August. On 3 and 6 August, US atomic bombs were dropped on Hiroshima and Nagasaki, and on 15 August Emperor Hirohito, effectively announced an end to hostilities. The formal surrender was signed on 2 September. As a result of the ceasefire, the planned Allied invasions of Japan were no longer required and the strategic gains provided by the capture of North Borneo were rendered useless in the military sense. The planned development, designed to facilitate the amphibious assaults on the Japanese home islands, included the creation in Brunei Bay of a major naval base, and this was cancelled. To some extent, this has led to some Australian claims that the 'Oboe' operations and the campaigns in the Aitape and Wewak region of New Guinea, and on Bougainville and New Britain islands, had been unrequired and had therefore been the cause of needless losses. The fighting in North Borneo cost the Australians 114 men killed or died of wounds while another 221 men were wounded from a committed total of between 29,000 and 30,000 men. The Japanese lost at least 1,234 men counted killed out of a total of some 8,800 men, while 130 had been captured. Moreover, a further 1,800 Japanese were estimated to have been killed by the guerrilla forces operating in the interior; many of these were Japanese troops who were withdrawing inland following the conventional landings on the coast who were ambushed by guerrillas or attacked by Allied air attacks summoned by these forces, which had also occupied large areas in Sarawak and the southern parts of North Borneo by the end of hostilities.

After the end of the fighting, the Australians embarked on the task of re-establishing the British civil administration, rebuilding the infrastructure which had been damaged and providing for the civilians who had been displaced in the fighting. This proved to be a major undertaking in which the 9^th Division worked to create hospitals, dispensaries and schools. Sanitation and drainage had not been provided by the Japanese, and the local population was both suffering from disease and malnourishment. Infrastructure was rebuilt by Australian engineers, while 9^th Division’s medical personnel provided direct aid to the local population. The 82-mile (132-km) North Borneo railway was also re-established, and houses that had been destroyed in the Japanese pre-invasion bombardment and in subsequent fighting were also rebuilt.

Following the ceasefire, there were still large numbers of Japanese troops in North Borneo, it being estimated that by October there were still more than 21,000 Japanese troops and civilians, and the 9^th Division was charged with the organisation of Japanese surrender, and the feeding and protection of these personnel. The division was also tasked with liberating the Allied civilian internees and prisoners of war held at Batu Lintang camp in Kuching, Sarawak, and with the disarming of the the guerrillas who had been assigned to the 'Agas' and 'Semut' operations.

As the civil administration was steadily restored, in October process of Australian demobilisation started. At first this process was slow, for there were few troops able to relieve the Australian forces in Borneo and therefore only long-service men were released for return to Australia. The 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. Most of the 9^th Division’s men returned to civilian life, but as part of Australia’s contribution to the Allied occupation of Japan, a number of the 9^th Division’s men were transferred to the 67^th Battalion, which was being formed as part of the 34^th Brigade.