'Oboe I' was the Australian seizure of Tarakan island in Japanese-occupied Borneo within the overall scheme of the 'Montclair' plan (1 May/24 June 1945).

'Oboe I' was the first of three operations, out of a planned six to be undertaken in the East Indies, to be completed by Lieutenant General Sir Leslie Morshead’s Australian I Corps in Borneo, whose recapture was deemed essential by the Allied high command for the economical prosecution of the war in the Pacific theatre commanded jointly by General Douglas MacArthur of the South-West Pacific Area and Admiral Chester W. Nimitz of the Pacific Ocean Areas.

Tarakan is an oil port on an essentially triangular island about 16 miles (25.75 km) long and 11 miles (17.75 km) wide in the delta of the Sesayap river of eastern Borneo, with production in the World War II period of about 5.1 million barrels per year from two fields near the centre of the island. The port’s facilities were strictly limited except for its refinery and four associated oil-loading piers, which were located at Tarakan town on the south-west coast. There was a small airstrip about 1 mile (1.6 km) to the north-west of the town, and this had played an important part in the Japanese 'B' (ii) offensive against Borneo and subsequent advances into the Netherlands East Indies. There was also a seaplane base with two underground fuel tanks each capable of holding 220,000 US gal (832780 litres) of fuel. In World War II the island’s interior was heavily forested, and much of the coast is mangrove swamp except in the area round Tarakan town. The interior comprises rugged hills reaching to a height of 100 ft (30 m), and island’s population was about 7,000 persons.

Tarakan island fell to the Japanese on 12 January 1942, and the victors immediately moved elements of the Imperial Japanese navy’s 'Tainan' Kokutai onto the airfield to provide cover for advances farther to the south.

The oil of Tarakan is of the light sour crude variety, and contains enough in the way of volatile fractions that it could be used as fuel for ships without any refining, but it also contained enough sulphur to render iron boilers brittle when so used. By 1944, the Japanese navy was sufficiently desperate for fuel that it began to use raw Tarakan crude oil despite the consequent risk of serious boiler damage.

The Japanese invasion force, led by Rear Admiral Sueto Hirose, commander of the 22^nd Base Force, comprised 16 transport vessels, four minesweepers, and two seaplane tenders carrying the 2^nd Kure Special Naval Landing Force, the 2^nd Base Force, and elements of Major General Shiguo Sakaguchi’s 56^th Independent Mixed Brigade. Air cover was provided by the 21^st Air Flotilla, and the transport vessels were covered by a strong escort force of four light cruisers and 15 destroyers. This force departed Davao on 9 January 1942 and arrived off Tarakan on the following day.

Tarakan was the first significant Dutch territory invaded in the Pacific War of World War II, and Japan’s formal declaration of war on the Netherlands was delayed until the day of the invasion, although the Dutch had declared war on Japan on 8 December 1941. The delay in the Japanese declaration of war may have been intended to keep the Dutch from demolishing their oil fields before the Japanese could seize them.

The Dutch defenders comprised the 7^th Garrison Battalion and some pieces of coastal artillery, but managed to sink the minesweepers W-13 and W-14 before being overwhelmed on the following day. As the Dutch commander had already offered to surrender at the time the minesweepers were sunk, the Japanese executed 219 of the artillerymen, who had opened fire as they had not yet received the order to surrender.

Seven Boeing B-17 Flying Fortress heavy bombers attacking from Malang encountered bad weather, forcing four of them to turn back. The other three scored no hits.

The invasion force was much stronger than necessary for this operation, but the Japanese intended to continue working the force to the south to become the Central Force for the 'J' (ii) invasion of Java, the ultimate objective of the 'Centrifugal Offensive'.

Tarakan’s oilfields were sabotaged by the Dutch before their surrender, but the Japanese were able to restore them to full production with 350,000 barrels per month flowing by a time early in 1944.

For the rest of World War II, the great island of Borneo was held and defended by Lieutenant General Masao Baba’s 37^th Army with the equivalent of 16 battalions (two brigades and one regiment, totalling 31,000 men) supported by two naval units, and these were deployed as Major General Taijiro Akashi’s 56^th Independent Mixed Brigade at Tawau on the coast of British North Borneo close to the border of the British and Dutch colonies in Borneo, Major General Hyoe Yamamura’s 71^st Independent Mixed Brigade in the Balikpapan area of Dutch Borneo with detachments garrisoning the Bandjermasin area on the southern end of the island, the 25^th Independent Mixed Regiment at Jesselton in British North Borneo, and two garrison battalions at Kuching in western Sarawak. The elements of the Japanese navy on the island, under the command of Rear Admiral Michiaki Kamada, included the 22^nd Special Base Force at Balikpapan on the central part of Japanese-occupied Dutch Borneo’s eastern side, and the 2^nd Guard Force on Tarakan island off the north-east coast of Dutch Borneo; both the naval units were reinforced by army personnel.

Borneo’s most important asset, so far as both the Allies and Japanese were concerned, was oil in fields whose recapture by the Allies would not only deprive the Japanese of their output, but also provide the Allies with a ready source of fuel for the offensives planned against the Japanese home islands.

The Australian operations in Borneo were planned in conjunction with the US 'Victor' operations in the southern part of the Philippine islands group, and became feasible with the gathering of the Australian I Corps on Morotai island of the Halmahera islands group of the Moluccan archipelago of the Netherlands East Indies from 22 February 1945. Though the US Joint Chiefs-of-Staff (and especially Admiral Ernest J. King) wished the operations of the Australian I Corps to be supported by Admiral Sir Bruce Fraser’s British Pacific Fleet, Nimitz vetoed the scheme as he needed the British aircraft carriers for 'Iceberg', in which the armoured flight decks of the British carriers proved far better able to cope with Japanese kamikaze attacks than the unarmoured decks of the more numerous US carriers. Thus the 'Oboe' operations were afforded US Navy support.

'Oboe I' was designed to wrest Tarakan island and its oilfield from a Japanese strength of some 1,700 troops (Captain Saichiro’s 455^th Independent Battalion and Commander Kunio Mizoguchi’s 2^nd Guard Force) and 400 civilians under the overall command of Major Tadai Tokoi.

The importance of Tarakan to the Japanese had disappeared with the rapid advance of Allied forces into the area. The last Japanese oil tanker left Tarakan in July 1944, and heavy Allied air raids later in the year destroyed the island’s oil production and storage facilities. In line with its declining importance, the Japanese garrison on Tarakan was reduced early in 1945, one of the two infantry battalions being withdrawn to Balikpapan island, where it was destroyed by Major General Edward J. Milford’s Australian 7^th Division in 'Oboe II'.

The primary objective for 'Oboe I' was to secure and develop the island’s airstrip in order to provide air cover for the subsequent Australian landings in Brunei and Balikpapan. The secondary objective was to secure Tarakan’s oilfields and bring them into operation as a source of oil for the Allied forces in the theatre.

Before the arrival of the invasion force, the Japanese garrison on Tarakan was subjected to intensive air attacks in the period 11/29 April. The first elements of the invasion fleet, Rear Admiral Russell S. Berkey’s Task Group 74.3 (Tarakan Attack Group) of Vice Admiral Thomas C. Kinkaid’s US 7^th Fleet sailed from Morotai island on 30 April carrying Brigadier David A. Whitehead’s Australian 26^th Brigade Group reinforced to an overall strength of 12,000 men by engineer and support troops as well as 400 Dutch in the form of one company of the Dutch Insular Corps and administrative personnel.

The invasion force passed to the north of Celebes and reached a point off Tarakan island on 27 April, where some of the vessels began operations to clear the minefields which the Allies had laid in their effort to prevent the Japanese shipping Tarakan’s oil.

On 30 April the 2/4^th Commando Squadron and 57^th Battery of the 2/7^th Field Regiment were landed on the nearby Sadau island in order to support the 2/13^th Field Company tasked with clearing the obstacles off the invasion beaches. The 26^th Brigade Group was landed on 1 May by the US 6^th Amphibious Group, whose Task Group 74.3 included the US command ship Rocky Mount, Australian infantry landing ships Manoora and Westralia, US attack cargo ship Titania, US dock landing ship Rushmore, 21 tank landing ships, 12 infantry landing craft, four medium landing ships and 12 tank landing craft, supported by six support landing craft, four rocket-armed infantry landing craft, two medium infantry landing craft, and two demolition infantry landing craft carrying four demolition units, and screened by the US destroyers Waller, Bailey, Bancroft, Philip, Drayton, Smith and Caldwell, Australian frigates Burdekin, Barcoo and Hawkesbury, US destroyer escorts Formoe and Charles E. Brannon, and 12 motor torpedo boats based on the US tender Wachapreague.

The landing was aided by a heavy air attack and the covering fire of three cruisers and six destroyers of Kinkaid’s 7^th Fleet. This Task Group 74.3 comprised the US light cruisers Phoenix and Boise, Australian light cruiser Hobart, US destroyers Taylor, Nicholas, O’Bannon, Fletcher and Jenkins, and Australian destroyer Warramunga.

The Australian 2/23^rd and 2/48^th Battalions came ashore at about 08.00 and cleared the Japanese coastal defences with relatively light casualties. Most of the brigade’s remaining combat units (including C Squadron of the 2/9^th Armoured Regiment with Matilda II tanks) were landed later on 1 May. After securing the beach-head, the 26^th Brigade Group advanced east into Tarakan town and north toward the airstrip. Overcoming stiff Japanese resistance, the Australians captured the airstrip on 5 May, and the town had been largely secured by 6 May.

While the capture of the airfield achieved the 26^th Brigade Group’s main task, the Japanese still held Tarakan’s rugged interior. In order to secure the island and protect the airstrip from attack, the 26^th Brigade Group was forced to clear the Japanese from well prepared positions in the heavily forested hills. While this necessarily entailed costly infantry fighting, the Australian troops made extensive use of their available artillery and air support, intensive ground-attack operations being flown by Air Commodore Harry Cobby’s (later Air Commodore Frederick Scherger’s) 1^st Tactical Air Force of the Royal Australian Air Force and Major General Paul B. Wurtsmith’s US 13^th AAF. The Japanese garrison was gradually destroyed, with the few survivors abandoning the remaining positions in the hills and withdrawing north of the island on 14 June, and from here they attempted to escape to the mainland of Borneo. The last organised Japanese resistance was encountered on 19 June, although mopping-up operations continued until the end of the war.

While the men of the 26^th Brigade Group fought the Japanese in the hills, engineers of the Royal Australian Air Force’s No. 61 Airfield Construction Wing were trying to bring Tarakan’s airstrip into operation. Because of the heavy damage it had suffered in the pre-invasion bombing, as well as its location in marshy terrain, the airstrip proved far more difficult to repair than had been expected and it took eight weeks, rather than the expected one week, to restore the strip to serviceability, and then only by extensive use of interlocking steel plates laid down like matting.

However, when the airstrip was reopened on June 28, the opportunity had passed for it to play any role in supporting the landings of the 'Oboe II' and 'Oboe III' operations. However, the RAAF’s No. 78 Wing was based on Tarakan from 28 June and flew in support of the continued 'Oboe II' operation until the end of the war. The invasion of Tarakan also liberated the civilian population from Japanese rule.

The 26^th Brigade Group remained on Tarakan as an occupation force until 27 December 1945, though most of its units were disbanded in October.

'Oboe I' cost the Australians 225 men killed and 669 wounded, while Japanese losses were 1,540 men killed and 252 taken prisoner. The remnants of the Japanese garrison managed to escape capture and moved toward the north-west to link with other Japanese garrisons in the area of Brunei Bay, where some 300 men survived to surrender at the end of the war.