Without the more than 50.000 G.I. of the US divisions sent in the French North Africa in Nov. 1942 there would be no Torch and no landings.
As Montgomery was unable, then and later, to destroy the German and Italian Armoured Armee war in North Africa would go on until, maybe, autumn 1943 (and, maybe,m with an AXIS victory, but surely without a landing in Sicily or anywhere in the Med. as the season was too much advanced) and Stalin, pressed by the majority of the Kremlin, would have made a separate peace with Hitler not beyond Sept. 1943 when USSR touched the lowest point of her internal situation.
Without 50 US Army divisions available no landing in Normany or in any other place in 1944 or later. With only a dozen of Divisions the British would not do a damn except to remain sitting on their island like in 1940, 1941, 1942 and 1943 (the same British Army was not mobilized, calling the workers from the factories, with a following drop in industrial output, before Autumn 1943).
Without the US Navy the conventional U boat menace would not be defeated during Spring 1943 (it was a matter of numbers of escorts at sea. The British had a great eperience, doctrine ect., but number only annihilate and the Royal Navy had the strenght to do only an half of the job).
No the Yanks were perhaps not brilliant on the ground (the landings in Marocco, Kasserine, Gela, the Gustav line, Anzio, the Ardennes are all milestones of military blunders while Korea and Vietnam are simply confirmations that ther's something wrong, like in Irak today) but without them the game was over (and I believe strongly the British would have been, after the loss of Singapore in Feb. 1942, only too much happy to do a bargain with their German cousins, so racially minded like them, at the expenses, if possible, of the goddamed frogs and of the dagos).