'Culverin' was a British unrealised plan for an amphibious assault against the northern end of Sumatra in the Japanese-occupied Netherlands East Indies, presupposing the capture of the Andaman and Nicobar island groups (dry season 1944/45).
The undertaking was eventually ruled out as it would have needed forces three times larger than those already required for planned Burma operations, and at a time when all amphibious transport was in short supply in the South-East Asian and related campaign because of the higher-priority requirements for operations in the Pacific, the Mediterranean and the English Channel. The operation was also rendered superfluous by the Japanese surrender in August 1945.
Had the 'Culverin' operation gone ahead and succeeded, it would have opened the way for an attack on the Kra isthmus for an advance on Bangkok, and for an attack to the south along the Strait of Malacca with a view to recapturing Malaya and/or Singapore.
The concept for 'Culverin' was first mooted by Prime Minister Winston Churchill at the 'Quadrant' conference in Quebec on 20 August 1943. Churchill was dissatisfied with the limited scope of current Allied plans for the South-East Asian theatre for 1943/44, and felt that by taking northern Sumatra 'we should be striking and seizing a point of our own against which the Japanese would have to beat themselves if they wished to avoid the severe drain which would be imposed on their shipping by our air action from Sumatra'. No detailed staff study of such an operation had been undertaken up to this time, and the matter was allowed to lapse before being revived in February 1944, when a delegation from Admiral Lord Louis Mountbatten, Supreme Allied Commander in South-East Asia, reported to the Defence Committee in London.
Mountbatten proposed amphibious operations in co-operation with the US-led forces of General Douglas MacArthur’s South-West Pacific Area command. 'Culverin' would be a necessary first part of this plan. The US Joint Chiefs-of-Staff did not favour this idea, however, in part because Mountbatten’s deputy, the US Lieutenant General Joseph W. Stilwell, had sent a separate mission to Washington to present his own somewhat differing views. In the event, most of the Japanese navy’s battleships and cruisers were transferred to Singapore at about the same time, radically strengthening the capability of the Japanese to react swiftly and with great strength to any Allied move against Sumatra. As amphibious operations depended on local naval superiority, 'Culverin' had therefore to be abandoned as the Royal Navy could not reinforce its forces in the Indian Ocean with sufficient strength to face the Japanese main battle fleet, and the US Navy did not favour operations in what was clearly a secondary theatre.
The South-East Asia command was considerably more cautious than Churchill in its estimates for the resources required for successful amphibious operations. For the much less ambitious 'Buccaneer', as the proposed capture of the Andaman islands group was termed, the South-East Asia command suggested a land force of 50,000 men for a task which Churchill had assumed to be within the grasp of the 14,000 men of one division.
Moreover, as no land-based air support would have been available until a very large lodgement had been secured, Mountbatten demanded the use of almost every aircraft carrier possessed by the Royal Navy, which would have had adverse effects on the conduct of other operations.
In the middle of 1945, the matter of a Sumatran landing was finally dropped. The Japanese fleet had been destroyed, but the preferred plan was now 'Zipper' for a landing on the coast of Malaya.