What if Operation Sledgehammer was carried out in Autumn 1942 and the Allies landed on mainland Europe and actually manage to take Cherbourg and the rest of the Contentin peninsula(with whatever they needed to do that, the maximum amount they could gather in that time)and then waited out.
I think its widely understood that this would utterly fail and mean the loss of many men and supplies, extending the war on the Western fronts.
What would happen in Europe? I'm especially interested in the future of Barbarossa(would the Germans break earlier or would they receive reinforcements now that the threath from the West is less?), the future abilities of the UK and USA in Northern Africa(also that of Vichy France without Torch) and the effect of an Italy who's defeat is prolonged.