When Marshall pressed for Sledgehammer he didn't actually have the forces he was promising to provide.
According to Andrew Roberts in Masters and Commanders, Operation Sledgehammer called for a landing by an Allied Force of 9 divisions on the Cherbourg Peninsular which could capture Cherbourg and its ports and be gradually strenghtened to ensure the Allied presence on the continent and draw the Germans into Normandy.
The process for a cross channel invasion called for the build up of 500,000 US troops to be in the UK - Operation Bolero - followed either by Operation Sledgehammer - as describe above - or Operation Roundup - a 48 division landing between Bolounge and Le Harve pushing up into Belguim.
When Marshall presented these plans to Brooke and the British Chiefs of Staff he could only offer 2 and a half divisions and 700 planes, and by the time Marshall wanted Sledgehammer put in operation - September 1942 - there were only 15 and a half US divisions earmarked for European service in existance and they were in the States. And at that period of time the US could provide no significant airforces for the operation, nor shipping, and the British themselves could not provide extra airforces or shipping due to commitments to the Middle-East, North Africa, India and Burma.
In short, Sledgehammer was impracticable to all involved, and Marshall knew that when he presented it. He didn't actually expect an agreement to get it put into effect, he expected an agreement in principal for a cross-channel invasion by 1943 at the latest.
The British were not prepared to outrightly refuse the American's plans for fear that they would take their ball and go home, that is head to the Pacific and abandon the "Germany First" agreement if the British didn't agree to do what the US wanted, and though Brooke, Churchill, Portal and Pound agreed with Bolero wholeheartedly and agreed in principal to a return to the continent by 1943 they did not agree with either Sledgehammer or Roundup - yet Marshall took Churchill pomposity and bluster as full support for both operations even though Brooke and the Chiefs of Staff had presented major misgivings over them.
When the British Chiefs of Staff flew to America later that year, Churchill met Roosevelt without them in New York while Brooke, Portal and Pound met with the Marshall and the American Chiefs of Staff. At this time Brooke and Marshall both agreed to support Bolero and a cross channel invasion in 1943 while Churchill and Roosevelt agreed to support Operation Gymnast - the invasion of North Africa which became Torch - mainly because Roosevelt wanted Americans in action against the Germans before the next election, and so for political reasons Bolero was to be delayed and the cross channel invasion put back a further year. Marshall did not accept this and continued to argue against it.
Later Marsall would fly back to the UK with King and Hopkins to get a definite decision of where the American troops would be used. Roosevelt told him that if he couldn't get the British to agree to Sledgehammer then he would have to find some alternative operation - by that time there was no doubt that Roosevelt was favouring North Africa.
Marshall was prepared, at that time, to use the threat of transfering the bulk of American manpower and material to the Pacific to deal with Japan if the British didn't agree to cross the channel and he and King had written up an ultimatum about this which they presented to Roosevelt to confirm. Roosevelt rejected it however saying that he did not like the "Japan First" memorandum because struck him like "taking up your dishes and going away" - a childish and petulant response for not getting their own way - nevertheless it did show that Marshall was prepared to use the Pacific as a threat to made the British fall in line.