'Sledgehammer' was an Allied (primarily US) unrealised plan for a landing of limited aims on the Cotentin peninsula of German-occupied northern France together with its major port at Cherbourg, or the western tip of the Brittany peninsula with its port of Brest (autumn 1942, later amended to 1943).

The operation was strongly urged by the US military establishment and the USSR, but rejected by the British, who believed that any landing in France was premature and thus impractical. As a result, 'Sledgehammer' was never implemented, and the British instead proposed an invasion of French North-West Africa, which took place in November 1942 as 'Torch'.

After the Japanese 'Ai' carrierborne air attack on the US Pacific Fleet at Pearl Harbor in December1941 precipitated the USA into World War II as an active combatant, the US Joint Chiefs-of-Staff pressed for an invasion of mainland Europe via the English Channel at the earliest possible moment. On 9 March 1942, in a letter to Prime Minister Winston Churchill, US President Franklin D. Roosevelt wrote: 'I am becoming more and more interested in the establishment of a new front this summer on the European continent, certainly for air and raids. From the point of view of shipping and supplies it is infinitely easier for us to participate in because of a maximum distance of about three thousand miles. And even though losses will doubtless be great, such losses will be compensated by at least equal German losses and by compelling the Germans to divert large forces of all kinds from the Russian front.'

On 8 April, General George C. Marshall and Harry Hopkins, the US Army chief-of-staff and Roosevelt’s personal emissary respectively, arrived in the UK to press the case for two possible US plans for a landing in occupied France. These were 'Round-up' and 'Sledgehammer'. The 'Round-up' plan was the original Allied plan for the invasion of continental Europe, was to be mounted before April 1943 and to be executed by 48 divisions, of which 18 were to be British. The 'Sledgehammer' plan was designed to capture the French sea ports of Brest or Cherbourg during the early autumn of 1942 if either Germany or the USSR was on the verge of any military or political collapse. 'Sledgehammer' was to be carried out mainly by British troops as the Americans could only supply two or three trained divisions at that time. Churchill responded that it was 'more difficult, less attractive, less immediately helpful or ultimately fruitful than Roundup'. After capturing Cherbourg and areas on the Cotentin peninsula, on whose northern coast Cherbourg is situated, the beach-head was to be defended, held and enlarged into a lodgement through the winter of 1942 and into 1943 while troops were massed for a break-out in the spring of 1943. Hopkins added additional political weight to the proposal by suggesting that if US public opinion had anything to do with it, the war effort would be directed instead against Japan should an invasion of mainland Europe not soon be mounted.

However, the elements required for 'Sledgehammer' were not currently available: these were air superiority, amphibious warfare equipment, sufficient forces and adequate quantities of weapons and supplies. Despite this, the Joint Chiefs-of-Staff considered 'Sledgehammer' to be feasible.

Had 'Sledgehammer' been attempted, the British could have landed a mere six divisions at most, while the Germans had between 25 and 30 divisions in western Europe. On the assumption that it could actually have been established, a beach-head and lodgement on the Cotentin peninsula would have rapidly have been isolated and taken under attack by land, sea and air. The only suitable port, Cherbourg would no doubt have been mined, and aircraft and artillery would certainly have been employed for an immediate attack on the port city in strength while the German armoured forces were brought to bear.

The pressure to mount ''Sledgehammer' increased further when the Soviet foreign minister, Vyacheslav Molotov, arrived in the UK to press for the Anglo-American 'second front'. After trying and failing to persuade Churchill, Molotov travelled to Washington where he enjoyed a better reception and received more support for his request. Molotov then returned to London and was convinced that a second front in 1942 was actually part of Anglo-American policy.

British officials continued to press for action in North Africa, which would allow relatively inexperienced US forces to gain experience in a less risky theatre, and the also serve to permit a gradual build-up of overwhelming Allied strength before Germany was engaged head on. At the 'Argonaut' second Washington conference in June 1942, Roosevelt and Churchill decided to postpone the invasion across English Channel until 1943 and to make the first priority the opening a second front in North Africa. At the Second Claridge Conference in London on 20/26 July, Churchill and Roosevelt’s aide 'Hopkins agreed to substitute 'Torch' in French North-West Africa for US reinforcement of the Western Desert campaign.

Senior US commanders expressed strong opposition to the 'Torch' landings and after the Allied Combined Chiefs-of-Staff met in London on 30 July, Marshall and Admiral Ernest J. King, the US chief of naval operations, declined to approve the plan. Marshall and other US generals continued to advocate 'Sledgehammer', which the British rejected. After Churchill further pressed for a landing in French North-West Africa in 1942, Marshall suggested instead to Roosevelt that the US abandon the 'previously agreed Germany first' strategy and take the offensive against the Japanese in the Pacific theatre. Roosevelt’s response was that this would do nothing to aid the USSR. With Marshall unable to persuade the British to change their minds, Roosevelt ordered that 'Torch' was to have precedence over other operations and was to take place at the earliest possible date: this was one of only two direct orders he gave to US military commanders during the war. 'Torch' met the British objective of securing victory in North Africa and the US objective of engaging in the fight against Germany, albeit on only a limited scale.

In the interim, a large-scale Canadian-led raid on the French coast was planned as 'Jubilee' to take some of the pressure off the USSR. In November 1942 Lieutenant General Dwight D. Eisenhower, the commander of the US European Theater of Operations, told Churchill that no major operation on the European continent could be carried out before 1944.