'Sealion' was an Allied unrealised contingency plan for the evacuation of Lieutenant General Sir Richard McCreery’s British X Corps from the northern sector of the 'Avalanche' beach-head at Salerno for rapid movement to reinforce Major General Ernest J. Dawley’s US VI Corps in the same beach-head’s southern sector (13 September 1943).

Evolved only in outline form, the plan resulted from the anxiety and even the loss of nerve felt by Lieutenant General Mark W. Clark’s US 5^th Army staff about the continued viability of the beach-head in the face of determined counterattacks by Generaloberst Heinrich-Gottfried von Vietinghoff-Scheel’s 10^th Army and the continued Luftwaffe raids on the beach-head and the Allied ships supporting it.

The operation was planned with great reluctance by the British Commodore G. N. Oliver under the supervision of the US Vice Admiral J. Kent Hewitt, both naval commanders appreciating that any evacuation would be costly in the extreme. The commander of the Allied 15^th Army Group, General the Hon. Sir Harold Alexander, arrived in the beach-head on 15 September and ordered an immediate end to the preparations, which McCreery had all along believed to be wholly unnecessary and even counter-productive.

The reciprocal undertaking would have been 'Seatrain' for the US VI Corps to be shifted to the north to aid the X Corps.