America was part of the Allied forces and that will never be denied. It is also a matter of record that America only got involved after being attacked by Japan. Germany and Italy then joined forces with Japan which is what drew America into the war. Up to this point, America only supplied the UK and the rest of Europe with weapons and charged exorbitant prices for the arms supplied, and that is before we consider the Lend-Lease agreement which was stacked heavily in America's favour.
I agree that America made the difference between victory and defeat, but not until Japan's direct attack on America. Until that point, America was happy to remain neutral and watch the UK and Europe go down the pan. When America joined the war America needed the existing Allied forces as much as the existing Allied forces needed America.
If the war in Europe had been lost it is highly likely that America would have been defeated by Japan and their Allies. It was the unification of forces that won the war, not the Americans. Maybe we should thank Japan for its actions against America that unified the rest of the world, if we had remained fragmented the free world that we all take for granted would not exist. So don't take the high ground on this subject. America did not get involved by choice, America only got involved after being forced to after Japan's actions at the time.