No less an authority than England's prime minister at that time, Winston Churchill, expressed his belief after the attack on Pearl Harbor. "After the United States entered the war I never slept as soundly as the night following Pearl Harbor. For I knew that The American Race would now be entering the war and it would never be the same.”
Churchill knew that Germany was going to declare war on the United States in support of the Japanese attacking Pearl Harbor. He also knew that Germany had no means of defeating either Great Britain or the United States in an all-out war and that Japan joining the Axis alliance was not going to change that imbalance of resources.
Neither Adolf Hitler nor Hideki Tojo had the education or imagination to understand that, once the United States took sides in a conflict, it could not be stopped by any major power that controlled less resources then America could provide. And that the Americans, as they showed in their own civil war, were utterly relentless when they were angry enough to commit all their strength.