The Soviet military had a lot to do with breaking the Nazis in WWII. That Military enjoyed the benefit of Lend lease, particularly in US Trucks.
The US wasn't in Italy by itself, there were two armies, as well as Mark Clark's there was Monty's Eight Army with British, Canadian, Australian and Indian troops.
In the West Operation Overlord was a joint US, British and Canadian venture with other Empire and Occupied Europe troops. The US side of it was not as well prepared as it could have been because they dismissed advice from the British regarding beach clearing techniques (cf Hobart's Funnies for example) and they thought they could get by on one dock.
Just to look at Operation Overlord British, Empire and Allied Occupied Europe troops numbered about 84,000 (or which nearly 62,00 were British). The US First Army had 73,000 men. The Royal Navy and RAF were also kind of involved.
The whole thing was an allied effort. The US couldn't have done it on its own, Britain and the Empire could hold the Nazis off, but couldn't liberate Europe, and without aid may have had to seek peace or starve, the Soviets had the manpower, but lend lease helped them get some breathing space and oncentrate their manufacturing effort, and thus they broke the German Army.
It was not the US alone however