This 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 142nd Regiment and the 2nd 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 37th Army, now headquartered at Melalap due inland from the northern end of Brunei Bay. This army comprised Major General Taijiro Akashi’s 56th Independent Mixed Brigade (371st Battalion on Labuan island, 366th Battalion and 367th Battalion in Brunei, and 368th Battalion at Beaufort), the 25th Independent Mixed Regiment, and detachments of the Japanese navy’s 22nd Special Base Force and 2nd Guard Force.
At the start of ‘Oboe VI’, elements of the 56th 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 25th 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.
Assembled on Morotai island, Major General George F. Wootten’s 9th Division (less its 26th Brigade 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 9th 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 7th 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 6th 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 24th Brigade Group then landed on Labuan island and Brigadier W. J. Victor Windeyer’s 20th Brigade Group at Brooketon.
In overall terms, the 24th 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 24th Brigade Group then landed two battalions on the eastern side of Brunei Bay to take Beaufort on 28 June; and meanwhile the 20th 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 20th 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 24th 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 24th 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 24th 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 20th 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 9th 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.
The Australian losses in ‘Oboe VI’ were 114 men killed and about 221 wounded; five US sailors manning landing craft were also killed. Some 1,234 Japanese dead were counted, but large numbers died in the hills, largely as a result of attacks by local head-hunters, and some 130 Japanese were taken prisoner.