Operation Sledgehammer was a World War II Allied plan for a cross-Channel invasion of Europe, as the first step in helping to reduce pressure on the Soviet Red Army by establishing a Second Front, developed as a contingency alternative to Operation Roundup, the original Allied plan for the invasion of Europe in 1943. Allied forces were to seize the French Atlantic ports of either Brest or Cherbourg and areas of the Cotentin Peninsula during the early autumn of 1942, and amass troops for a breakout in the spring of 1943.
The operation was eagerly pressed for by both the United States military and the Soviet Union, but rejected by the British, who felt a landing in France was premature, and hence impractical.  This perception was reinforced by the failure of the smaller Dieppe Raid in August 1942. As a result, Sledgehammer was never carried out, and instead the British proposal for an invasion of French North Africa took place in November 1942 under the code name Operation Torch.
After the United States entered World War II, the U.S. Joint Chiefs of Staff pressed for an invasion of mainland Europe via the English Channel "as soon as possible", i.e. the early part of 1942. The British were, however, reluctant, as it was felt that other places had a higher priority, the time was not right and insufficient men and landing craft were available. British officials pressed for action in North Africa which would allow relatively inexperienced American forces to gain experience in a less risky theatre while gradually building up overwhelming force before engaging Germany head on.
The U.S. tended to regard this reluctance as an example of British caution but since at the time they lacked the resources to carry out such an operation themselves, the result was stalemate, along with increased pressure on the British, which began in March 1942 with a letter from President Roosevelt to Winston Churchill:
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.— Roosevelt to Churchill, 9 March 1942
On 8 April, General George Marshall and Harry Hopkins arrived in Britain to press the case for two possible American plans for a landing in Occupied France, Operation Roundup and Operation Sledgehammer.
Roundup was to be executed by 48 Allied divisions, 18 of which would be British. It was to be mounted before April 1943.
Sledgehammer was a plan to capture the French seaports of either Brest or Cherbourg during the early autumn of 1942 in the event that Germany or the Soviet Union was at the brink of collapse. Sledgehammer was to be carried out mainly by American troops as the British could only supply two or three divisions in 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, the beachhead was to be defended and held through the winter of 1942 and into 1943, while troops were massed for a breakout operation to take place in spring 1943. This plan became popular and received the codename Sledgehammer. Hopkins added additional political weight to the proposed plan by opining that if U.S. public opinion had anything to do with it, the U.S. war effort would be directed instead against Japan if an invasion of mainland Europe was not mounted soon.
However, the elements required for such an operation were lacking, i.e. air superiority, amphibious warfare equipment, sufficient forces and adequate supply. Despite all this, the Joint Chiefs of Staff considered Sledgehammer feasible.
If Sledgehammer had been carried out, the British could have landed only six divisions at most, whereas the Germans had 25-30 divisions in Western Europe. Assuming it could be established in the first place, a beachhead on the Cotentin peninsula would be blocked off and attacked by land, sea and air. Cherbourg, the only suitable port would no doubt be mined, while aircraft and artillery would be expected to attack the town in strength, while German armoured forces were brought to bear.
The pressure to mount Sledgehammer increased further when Soviet Foreign Minister Vyacheslav Molotov arrived in the UK to press for a Second Front. After trying and failing to persuade Churchill, Molotov travelled on to Washington where he enjoyed a better reception and received more support for his requests. He then returned to London convinced that a Second Front in 1942 was an actual part of Anglo-American policy.
Churchill pressed for a landing in North Africa in 1942. U.S. Army Chief of Staff General George Marshall suggested instead to Roosevelt that the U.S. abandon the Germany-first strategy and take the offensive in the Pacific. Roosevelt "disapproved" the proposal saying it would do nothing to help Russia. With Roosevelt's support, and Marshall unable to persuade the British to change their minds, in July 1942 Operation Torch, the invasion of French North Africa, was scheduled for later that year.
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