The holocaust was never a reason to go to war. The US was aware of it there were articles in the NY Times before the war started but they were always buried in the back. If Germany hadn't declared war on us and without a reason like in WWI to declare, we would have fought in Asia first instead of Europe and by the time we had defeated the Japanese the Soviets would have been all the way to the Atlantic. The big question then is how would the cold war and post-war development change when the Iron Curtain was the English Channel?