Operation Transom was an attack by Allied forces against the Japanese-occupied city of Surabaya on the Indonesian island of Java during World War II.

Conducted by the British-led Eastern Fleet, the operation took place on 17 May 1944 and involved American and British carrier-based aircraft bombing the city's docks and an oil refinery.

It was undertaken to divert Japanese forces from the Allied landing on Wakde island off New Guinea and make use of the American aircraft carrier on its return voyage to the Pacific.

The warships involved in the operation sailed from Ceylon and refuelled in Western Australia before reaching the waters south of Java, where the carriers' aircraft were launched.

On the morning of 17 May two groups of Allied aircraft made a coordinated attack on Surabaya's port and several industrial facilities that took the Japanese by surprise.

[8] Royal Australian Air Force Consolidated PBY Catalina aircraft also periodically dropped naval mines in the entrance to Surabaya's port from August 1943.

[12] The plan adopted by the Allied leaders at the November 1943 Cairo Conference stated that "the main effort against Japan should be made in the Pacific", and that the Indian Ocean would be a subsidiary theatre.

[13] In January 1944 the Admiralty, the British government institution responsible for administering the Royal Navy, decided to substantially reinforce the Eastern Fleet.

This change was made to evacuate the fleet from its bases in the central Pacific, which had become vulnerable to American attacks, and concentrate it at a location with good naval repair facilities and ready access to fuel.

Somerville believed that his force would be unable to counter the Combined Fleet if it entered the Indian Ocean, and more air units were dispatched to protect Ceylon.

[16] The United States Navy also agreed to temporarily transfer the aircraft carrier USS Saratoga and three destroyers from the Pacific to augment the Eastern Fleet.

[18] Illustrious and Saratoga, accompanied by much of the Eastern Fleet, conducted a successful air raid against the Japanese-held island of Sabang in the NEI on 19 April as part of Operation Cockpit.

[20] One of the main differences was to substitute Grumman Avengers for Illustrious's usual air wing of Fairey Barracuda torpedo and dive bombers.

This change was made because Surabaya's defences were expected to be stronger than those at Sabang, and Somerville had decided to launch the aircraft 180 miles (290 km) from the city, which was beyond the Barracudas' effective range.

[23] Because of the distance to be covered from Ceylon and the Royal Navy's lack of experience in underway replenishment, the final plans for the operation involved the Eastern Fleet refuelling at Exmouth Gulf in Western Australia before striking Surabaya.

[6] The United States Army Air Forces' 380th Bombardment Group, which was based near Darwin in Australia, was to also bomb Surabaya on the night after the carrier raid to prevent the Japanese from dispatching aircraft to attack the Eastern Fleet as it withdrew.

1 Wing RAAF at Darwin to Exmouth Gulf to protect the Eastern Fleet while it refuelled and Australian and American maritime patrol aircraft were assigned to operate offshore.

[7] They claimed to have damaged ten ships, demolished both the Wonokromo oil refinery and the Braat Engineering Works and destroyed 16 aircraft and several buildings at an airfield.

[41] After the strike force completed landing on the carriers at 10:50 am, the Eastern Fleet withdrew to the south-west in an attempt to obscure the fact that it was headed for Exmouth Gulf.

As a result, they did not learn until photographs taken by one of Saratoga's photo reconnaissance aircraft were dropped onto Queen Elizabeth at 3:00 pm that many worthwhile targets, including Japanese submarines, remained in Surabaya's port.

[36] USS Rasher attacked a Japanese convoy near Ambon on 11 May while en route to take up station north of Surabaya, sinking one ship and damaging several others.

[44][45] The historian Clay Blair judged that the submarines would have likely achieved more if they had undertaken routine patrols or been stationed near the major Japanese naval anchorage at Tawi-Tawi.

Adelaide and one of the Australian destroyers that had been attached to the Eastern Fleet left Exmouth Gulf bound for Fremantle after the tanker group departed on 19 May.

[52] The Japanese leadership incorrectly interpreted the American landing at Biak off the north coast of New Guinea on 27 May as being the main Allied effort, and the Combined Fleet dispatched a powerful force to make a counter attack on 10 June in what was designated Operation Kon.

This attack was cancelled two days later when it became apparent that the Americans were about to invade the Mariana Islands in the Central Pacific, and the Combined Fleet was defeated during the Battle of the Philippine Sea fought between 19 and 20 June.

The official historian of the overall British effort in South East Asia, Stanley Woodburn Kirby, wrote in 1962 that the Wonokromo oil refinery and other industrial facilities were set on fire, the naval dockyard and two other docks were bombed and twelve Japanese aircraft destroyed on the ground.

[57] Jürgen Rohwer stated in 2005 that twelve Japanese aircraft were destroyed on the ground, a small freighter was sunk and a patrol boat damaged beyond repair.

[58] In 2011 David Hobbs judged that the operation was successful, the Wonokromo oil refinery being "burnt out", naval dock installations damaged and a merchant ship sunk.

[59] Hobbs identified other lessons the Royal Navy took away from the operation, including a need to plan to conduct at least two strikes against each target and the desirability of obtaining photo reconnaissance aircraft that could be flown from carriers.

[60] Gray also noted that the operation, and especially the need to refuel in Western Australia, illustrated to the Royal Navy that it required an improved ability to replenish warships while they were at sea.