'Oboe II' was the Australian seizure of Balikpapan, toward the southern end of Borneo’s eastern side, within the overall 'Montclair' plan (1/10 July 1945).

In 1941 Balikpapan, which lies on the Sumbir river on the south-east coast of Borneo, was the largest town in Dutch Borneo and boasted an important oilfield yielding 7.4 million barrels per year, a refinery at the terminus of the pipeline bringing oil from the two oilfields some 25 and 50 miles (40 and 80 km) away to the north-east, and a newly constructed port with the facilities for the loading tankers. Like that of most of Borneo, the climate is hot and wet, and marked by little in the way of seasonal variation. The airfield at Sepinggang, with a 5,000-ft (1525-m) runway, was located on the coast, while the airfield at Manggar was 13 miles (21 km) to the east of the town. At the time in question, the surrounding country was hilly and covered with jungle. There was a well-developed road following an oil pipeline to Samarinda, and this also provided access to the oilfields of the interior.

In December 1941, Balikpapan was protected by the 1,100 men of the 6^th Garrison Battalion as well as two 4.7-in (120-mm) and four 2.95-in (75-mm) pieces of coastal artillery, and the US Navy’s Destroyer Division 57 was in the harbour on its way to Singapore.

The task of escorting the Japanese assault force to Balikpapan was allocated to Rear Admiral Shoju Nishimura’s Destroyer Squadron 4 with the light cruiser Naka and the destroyers Harusame, Samidare and Yudachi (Destroyer Division 2), Asagumo, Murasame, Minegumo and Natsugumo (Destroyer Division 9), and Kawakaze, Umikaze and Yamakaze (Destroyer Division 24), an air group with the seaplane tenders Sanyo Maru and Sanuki Maru, one oiler, Hirose’s 2^nd Base Force with the patrol boats PB-36, PB-37 and PB-38, the minesweepers W-15 and W-16 (Minesweeper Division 11) and W-17 and W-18 (Minesweeper Division 30), the submarine chasers Ch-10, Ch-11 and Ch-12, the army transport vessels Ashiyama Maru, 3,519-ton Sumanoura Maru, 5,175-ton Kuretake Maru, Kumagawa Maru, Toei Maru and Yukka Maru carrying the 56^th Independent Mixed Brigade, and the navy transports 7,070-ton Tatsugami Maru, 6,987-ton Tsuruga Maru and 6,557-ton Nana Maru carrying the 2^nd Kure Special Naval Landing Force, seven other transport vessels, and elements of Rear Admiral Takeo Tada’s 21^st Air Flotilla.

Some 5,500 men of Major General Shizuo Sakaguchi’s 56^th Independent Mixed Brigade (scheduled to become the infantry group of the 56^th Division) and elements of the 2^nd Kure Special Naval Landing Force came ashore to take the town on 23/24 January 1942. The Allies' ABDA Command attempted to contest the landings with the few forces available, but it was a hopeless task. Nine Dutch Martin Model 139 medium bombers from Samarinda and three US Boeing B-17 heavy bombers from Soerabaja attacked the Japanese force as it was coming to anchor on the evening of 23 January, sinking the transport vessel Nana Maru and damaging another vessel. Of the eight submarines (two Dutch and six US) sent to the area, only the Dutch K-XVIII managed to reach the anchorage and penetrate the screen, sinking the transport vessel Tsuruga Maru just before 24.00. The commander of the Japanese screening force took his ships to sea in a hunt for the submarine, leaving the transports largely unprotected.

In the early hours of 24 January, the US destroyers Pope, John D. Ford, Parrott and Paul Jones, of Rear Admiral William A. Glassford’s US Task Force 5, were therefore able to slip into the harbour and rake the anchored transports with torpedoes and gunfire, sinking the transport vessels Sumanoura Maru, Kuretake Maru, Yukka Maru and Tatsugami Maru and the patrol boat PB-37, and severely damaging the transport vessels Kumagawa Maru and Toei Maru. This was one of the few successes gained by forces of the ABDA Command, though the US destroyers should in theory have been able to succeeded still better against anchored, silhouetted and largely unprotected transport vessels. Most of the Japanese assault troops had already been loaded into their landing craft, however, and the battle set back the Japanese time table by less than a day. By the evening of 24 January the Japanese had the main airfield ready for operations.

The Dutch garrison responded by firing the oil wells and retreating into the interior. The Japanese advance on the town itself was slowed by the demolition of the bridges, but by evening of 25 January the Japanese had taken the town without a fight. A second stealthy landing by a single Japanese battalion (guided by fifth columnists) encountered little opposition and cut off the retreat of a few Dutch stragglers, but failed to secure the oilfields before they had been fired.

The Japanese had despatched emissaries to the Dutch commander ordering the Dutch forces to preserve the oilfields or face summary execution, and on 20 February the Japanese made good on their threat, massacring every European (at least 72 persons) they could find in the vicinity of the town in retaliation for the destruction of the oil fields.

By 1945 the Japanese garrison amounted to some 4,200 men (including 1,100 Japanese and Formosan labourers) of the two-battalion 22^nd Special Base Force and Major Yamada’s 454^th Independent Battalion, and there were also 1,500 men at Samarinda, the location of the main oilfield, some 60 miles (100 km) to the north. The naval force manned a number of coast defence guns as well as anti-aircraft guns carefully concealed in the hills.

Representing the last Allied amphibious operation of World War II, the 'Oboe II' undertaking to retake Balikpapan posed some notably severe tactical difficulties for the 21,000 men of Major General Edward J. Milford’s Australian 7^th Division (Brigadier Frederick O. Chilton’s 18^th Brigade, Brigadier Ivan N. Dougherty’s 21^st Brigade and Brigadier Kenneth W. Eather’s 25^th Brigade), supplemented by another 12,000 support, service and base personnel contributed, along with the Matilda II tanks of the 1^st Armoured Regiment, by Lieutenant General Sir Leslie Morshead’s Australian I Corps. The Japanese were well emplaced in the hills overlooking the town, and the approaches from the sea had been liberally mined by both the Allies and the Japanese.

The Australian landings were preceded by 20 days of heavy air attacks and the heaviest pre-landing gunfire bombardment of the South-West Pacific Area campaigns. This ignited many of the oil storage tanks. US underwater demolition teams cleared many of the Japanese obstacles from 26 June onward. The prolonged bombardment had been prompted by intelligence reports which indicated the presence of strong fortifications in the area, and probably also a desire to minimise Australian casualties this late stage of the war. The strength of the Japanese fortifications was not illusory: the Japanese had constructed a large number of pillboxes, bunkers, tunnels and obstacles some 400 to 1,000 yards (360 to 915 m) inland, and had also excavated tank traps up to 14 ft (4.25 m) wide near the beaches. Log barriers had been constructed 70 to 100 yards (65 to 90 m) from the shore, and the area was heavily mined, including large numbers of Allied acoustic and magnetic mines dropped from aircraft earlier in the war.

The shallowness of the water, which was less than 60 ft (18.3 m) deep to a distance as far as 6 miles (9.6 km) offshore, meant that mine clearance could be undertaken only by smaller craft, and would be difficult to provide with gunfire support.

On 15 June a US group of three fleet minesweepers and 38 motor minesweepers began to clear in the extensive Japanese minefields in the waters round Balikpapan, which had also been liberally sown with US air-laid mines. In the course of an effort which lasted to 1 July, 18 US air-laid magnetic mines and nine Japanese moored mines were cleared, but the yard minesweeper YMS-50 was lost on 18 June, YMS-39 and YMS-365 on 26 June and YMS-84 on 9 July; YMS-368, YMS-47 and YMS-314 were damaged by mine detonations; and YMS-335, YMS-10, YMS-364 and YMS-49, as well as the destroyer Smith, were damaged by the fire of Japanese coastal batteries. However, since it was known that 93 Allied mines had been dropped (and none were set to self-deactivate) it was believed that there were still a considerable number of mines in the area. However, no other ships or boats were lost during the landings.

Cover and gunfire support for the minesweeping effort was provided by elements of Rear Admiral Ralph S. Riggs’s Task Group 74.2 with the light cruisers Denver and Montpelier, and four destroyers.

Underwater demolition teams arrived on 24 June to begin destroying the obstacles between the sea and the beaches. More than 300 yards (275 m) of obstacles were destroyed before the landings.

Delays in restoring the airstrip at Tarakan, taken in 'Oboe I', to operational condition once more meant that tactical air support had to be furnished at extreme range from Tawi-Tawi, with consequent range and endurance limits especially on the fighters and fighter-bombers, though the activities of Japanese aircraft led Kinkaid to request escort carrier support for his naval forces, a request emphasised on 25 June by an unsuccessful attack by Japanese torpedo bombers. Three US escort carriers were therefore provided to support the landings. These carriers had been operating off Kyushu in the Japanese home islands, but arrived on 1 July, the day of the landings, and provided air cover and support for three days.

The gunfire bombardment of the assault areas was continued from 25 to 30 June by TG74.2 with the light cruisers Denver, Montpelier, Columbia, Cleveland (carrying General Douglas MacArthur) and Dutch Tromp, and destroyers Conway, Eaton, Cony, Stevens, Albert W. Grant, Killen (Destroyer Squadron 44) and Australian Arunta; Rear Admiral Russell S. Berkey’s TG74.3 with the light cruisers Nashville and Phoenix (carrying Vice Admiral Daniel E. Barbey, commanding the Amphibious Forces of Admiral Thomas C. Kincaid’s US 7^th Fleet), and destroyers Charette, Bell, Conner and Burns(Destroyer Division 102); and Commodore H. B. Farncomb’s TG74.1 with the Australian heavy cruiser Shropshire and light cruiser Hobart, and US destroyers Hart and Metcalfe.

Ammunition was expended without stint in the pre-landing bombardment. Some 23,764 shells of 4.7-in (120-mm) calibre or greater were expended up to 1 July, another 11,884 on 1 July, and 11,158 more by 7 July. Another 114,000 rounds of 20- and 40-mm cannon ammunition were also expended. This exceeded by a considerable margin the ammunition expenditure of any other landing of divisional strength during the war.

As the gunfire bombardment ended, the first of some 33,446 troops (including two companies of the Dutch Insular Corps) of the reinforced 7^th Division from Morotai island were landed at Balikpapan on 1 July by Rear Admiral Albert G. Noble’s 8^th Amphibious Group (TG78.2, Balikpapan Attack Group) with the headquarters ship Wasatch, Australian infantry landing ships Manoora, Nimble and Westralia, one US attack cargo ship, one dock landing ship, five troop-carrying destroyer conversions, 35 tank landing ships, 22 medium landing ships and 50 infantry landing craft, escorted by the US destroyers Flusser, Drayton, Conyngham and Smith (Destroyer Squadron 5), Frazier, Bailey and Robinson (Destroyer Squadron 14), and Saufley, Waller and Philip (Destroyer Squadron 22), destroyer escorts Chaffee, Edwin A. Howard, Key, Leland E. Thomas and Rutherford, and Australian frigate Gascoyne. Eight US Navy PT-boats arrived with the tender Mobjack on 27 June, and this force was expanded to two PT-boat squadrons on 6 July.

Air support was provided by Major General Ennis C. Whitehead’s US 5^th AAF and Major General Paul B. Wurtsmith’s US 13^th AAF and, between 1 and 3 July, by the aircraft of Marine Air Group 2 operating from Rear Admiral William D. Sample’s TG78.4 with the escort carriers Suwanee, Gilbert Islands and Block Island), escorted by the destroyer Heim and destroyer escorts Cloves, Mitchell, Kyne, Lamons and Donaldson.

Some 10,500 men, 700 vehicles and 1,950 tons of supplies were brought ashore on the first day of 'Oboe II'. Once ashore, the 7^th Division moved inland with commendable speed only to run into stout Japanese defences. Naval gunfire support enabled the division to maintain its progress, however, and the Australians had taken Sepinggang airfield by 2 July, Balikpapan town by 3 July and Manggar airfield by 9 July, the Japanese pulling back into the hinterland in their standard fashion for guerrilla warfare. Because the end of World War II was within sight, the Australian commanders were under orders to minimise casualties and the advance towards Samarinda was therefore undertaken with great caution. The nearer oilfield was in Australian hands by 18 July, and by the end of the month organised resistance had ended, with the remaining Japanese retreating toward Banjarmasin and Kuching.

The Australian casualties were 229 men killed and 634 wounded, while the Japanese lost 1,783 men killed and 63 taken prisoner, the the survivors of the other 2,150 surrendering at the end of the war.