To be fair, America didn't really win WW2, we hardly even fought, mostly we just supplied Hitler with arms, and when that wasn't fashionable anymore, we supplied the allies with arms.. Then we swooped in at the end to take all the credit and the fat construction contracts.
In the 20s and the early 30s (that is to say, before Hitler rose to power), the US brokered efforts to reduce the burden of WWI reparations, as well as inject loans into Germany to help its moribund economy. FDR was opposed to Hitler, and would have brought the US into the war earlier, but US public opinion was firmly isolationist. Even then, the US was neutral really in name only, as it devised conditions for selling weapons that amounted to "only the UK can buy them." Yes, the US logistical support to the Allies was crucial both to keeping them collapsing and actually decisively winning the war, but the US military did prove superior to both Axis and other Allies in several regards, most notably artillery, amphibious assaults, and carrier operations.
