Hitler's biggest blunder was declaring war on the United States for no cause. No other nation except America could have bombed their war machine or posed enough of a threat to invade France to dilute his forces.
Wouldn't have really mattered...Britain declared war on Japan with us...So there went any restrictions we had on arming Britain...eventually the Nazis would have attacked our ships, and we would have had our Casus Belli for going to war with Germany.