Ultimately, it was Hitler and Mussolini’s declaration of war against the US on 11 December 1941, that overcame American opposition to a crusade against all the fascist powers.
Interestingly, Nazi Germany had signed no treaty that bound Hitler to support Japanese aggression. He was committed to taking on the US only in the event that America initiated hostilities.
What most drew Hitler into declaring war on the US was the very grandiosity of the thing. Not far beneath the surface, he was wildly excited that he was now the central figure in what had become the widest war in human history.