'Diplomat' was a naval undertaking by Admiral Sir James Somerville’s Eastern Fleet to prepare elements of the fleet in the operational procedures which would be required in planned operations against the Japanese in the Pacific as part of Admiral Sir Bruce Fraser’s British Pacific Fleet (created in November 1944), and also to protect shipping plying the route between Australia and India (21 March/2 April 1944).

The battleships Queen Elizabeth and Valiant, the battle-cruiser Renown, the fleet carrier Illustrious, the heavy cruisers London and Cumberland, the light cruisers Ceylon and New Zealand Gambia, and 10 destroyers departed Trincomalee and Colombo on 21 March to rendezvous on 24 March at a point some 850 miles (1370 km) to the south of Ceylon. The force now met three oilers, escorted by the Free Dutch light cruiser Tromp, and practised refuelling at sea for the next two days.

On 27 March the ships met the US Task Group 58.5 comprising the fleet carrier Saratoga and her escorting destroyers Dunlap, Cummings and Fanning. The joint force returned to Trincomalee, and Fleet Air Arm aircrew spent two days in learning the necessary procedures from US aircrews and seeking to digest the implications of the US aircrews' experience in fighting the Japanese.

The British ships involved in 'Diplomat' were the battleships Queen Elizabeth and Valiant, the battle-cruiser Renown, the fleet carrier Illustrious, the heavy cruisers Cumberland and London, the light cruisers Ceylon and New Zealand Gambia, and the destroyers Pathfinder, Quilliam, Quality, Queenborough, Australian Napier, Nepal, Norman and Quiberon, and Free Dutch Tjerk Hiddes and Van Galen.

On the same day, as a reinforcement for the escort carrier Battler, the escort carriers Atheling, Begum and Shah arrived and, soon after this, the Free French battleship Richelieu.

A secondary function of this operation was the protection of Allied shipping from Japanese interference while plying the route between India and Australia.