There were so few rifles available at the outbreak of the war our recruits were drilling with broomsticks. We leased a fleet of WWI destroyers to England to buy them some time as the Arsenal of Democracy geared up for war production, a valuable story in itself- Trump's interest in restoring American manufacturing reminds me of that.
Germany's best divisions were squandered on the Eastern Front about the time that we entered the war. If those troops had been available in June of 1944 we would have been shoved right back into the English Channel. The Russian winter and Hitler's insanity were valuable allies.
Well, and the bombing. Germany put 30% of its entire war resources into stopping US/Brit bombers. By June 44, it was 40% of all aircraft just used as interceptors.
I did a study of Kursk if the US had not come in, and with 30% more air (especially the tank-busting Stukas) Germany would have won Kursk.