Japanese forces invaded and conquered most of the Dutch East Indies between December 1941 and March 1942.
The island of Java was administered by the Imperial Japanese Army during the Japanese occupation of the Dutch East Indies.
Surabaya, which is located in eastern Java, was a naval base and industrial centre during the occupation.
Japanese anti-submarine forces based at Surabaya hunted Allied submarines operating in the Java Sea.
The Wonokromo oil refinery was important to the Japanese, and was the only facility in Java which produced aviation fuel.
The city's defences against air attack at the time of Operation Transom included a small number of anti-aircraft guns, whose crews were inadequately trained.

At the Cairo Conference held during November 1943, the Allied leadership agreed that "the main effort against Japan should be made in the Pacific", and that the Indian Ocean would be a subsidiary theatre.
It was also decided that any offensive operations, including aircraft carrier raids, in the theatre would have the goals of "maintaining pressure on the enemy, forcing dispersion of his forces, and attaining the maximum attrition of his air and naval forces and shipping".

The British Eastern Fleet, which was led by Admiral James Somerville and based in Ceylon, was heavily reinforced during early 1944.
The aircraft carrier HMS Illustrious, battleships HMS Queen Elizabeth and Valiant and battlecruiser HMS Renown arrived in January, providing the fleet with an offensive capability.
Many other ships arrived over the course of the year.

Also in early 1944, the Japanese military transferred its main naval striking force, the Combined Fleet, to Singapore.
This change was made to evacuate the fleet from its bases in the central Pacific, which were now vulnerable to American attacks, and concentrate it at a location with good naval repair facilities and ready access to fuel.
The Japanese did not intend to undertake any large-scale attacks into the Indian Ocean.
Somerville believed that his force would be unable to counter the Combined Fleet if it entered the Indian Ocean, and additional air units were dispatched to protect Ceylon.
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.

Saratoga and her escorts joined the Eastern Fleet on 27 March 1944.
Illustrious and Saratoga, accompanied by much of the Eastern Fleet, conducted a successful air raid against the Japanese-held island of Sabang in the Dutch East Indies on 19 April as part of Operation Cockpit.
Japanese forces invaded and conquered most of the Dutch East Indies between December 1941 and March 1942. The island of Java was administered by the Imperial Japanese Army during the Japanese occupation of the Dutch East Indies. Surabaya, which is located in eastern Java, was a naval base and industrial centre during the occupation. Japanese anti-submarine forces based at Surabaya hunted Allied submarines operating in the Java Sea. The Wonokromo oil refinery was important to the Japanese, and was the only facility in Java which produced aviation fuel. The city's defences against air attack at the time of Operation Transom included a small number of anti-aircraft guns, whose crews were inadequately trained.
At the Cairo Conference held during November 1943, the Allied leadership agreed that "the main effort against Japan should be made in the Pacific", and that the Indian Ocean would be a subsidiary theatre. It was also decided that any offensive operations, including aircraft carrier raids, in the theatre would have the goals of "maintaining pressure on the enemy, forcing dispersion of his forces, and attaining the maximum attrition of his air and naval forces and shipping".
The British Eastern Fleet, which was led by Admiral James Somerville and based in Ceylon, was heavily reinforced during early 1944. The aircraft carrier HMS Illustrious, battleships HMS Queen Elizabeth and Valiant and battlecruiser HMS Renown arrived in January, providing the fleet with an offensive capability. Many other ships arrived over the course of the year. 
Also in early 1944, the Japanese military transferred its main naval striking force, the Combined Fleet, to Singapore. This change was made to evacuate the fleet from its bases in the central Pacific, which were now vulnerable to American attacks, and concentrate it at a location with good naval repair facilities and ready access to fuel. The Japanese did not intend to undertake any large-scale attacks into the Indian Ocean. Somerville believed that his force would be unable to counter the Combined Fleet if it entered the Indian Ocean, and additional air units were dispatched to protect Ceylon. 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.
Saratoga and her escorts joined the Eastern Fleet on 27 March 1944. Illustrious and Saratoga, accompanied by much of the Eastern Fleet, conducted a successful air raid against the Japanese-held island of Sabang in the Dutch East Indies on 19 April as part of Operation Cockpit.
Operation Transom