From the beginning the Americans and the Soviets preferred a cross-channel invasion but the British were weary of a failure or a costly slog and thought that knocking Italy out of the war and going after the soft underbelly of Europe rather than the much more fortified French coast was more likely to succeed given the capabilities of the allies at the time. Particularly given the British manpower shortage and reliance on naval strength. The US and the British compromised on an eventual channel invasion but first knocking Italy out of the war. - Day of Battle, Rick Atkinson. So by 1943 it was already decided that there was going to eventually be a channel invasion by why did the success of the italian campaign not convince them they could break into Germany without a western front? The germans were able to fight a sort of limited war in Italy. The strain on the Wehrmacht was immense as the 400,000 troops in Italy were badly needed in the East and probably contributed to the failure of Operation Citadel but it didn't represent the same massive diversion of resources that were eventually needed to defend France. And while these troops were slowly being forced back in Italy they had done so relatively slowly and the successive defense lines in Italy were quite strong. So it was clear that to fully break through the German lines in Italy would require significantly more troops. If Italy was the main effort it had a few significant disadvantages to France. First, supplies had to be shipped significantly farther from the main supply centers in Britain to get to Italy. While the U-Boat threat was already on the decline and the Mediterranean bottlenecks made U-Boat operations difficult it was far less easy to secure than the channel in which U-boats and E-boats unsuccessfully tried to attack on D-Day and more importantly the distance itself meant less supplies could be landed. (although Italy's port capacity was significantly better than northern France) Second, the terrain in Italy favored defensive operation especially the Alps. Most of Italy is relatively mountainous and the peninsula is narrow allowing the Germans to negate some of the allied advantage in numbers by shortening the front. In comparison to the flat North Atlantic plain it was far harder to push. Lastly, the us did continue operations in Italy after the beginning of normandy allowing them to force the germans to spread the forces both to defend Italy and France.