DougR,
What Fionn said was that Hitler declared war on the USA rather than The USA declaring war on Germany. If Hitler hadn't done that, the USA would have been pushed to find a justifiable reason to declare war on Germany and would have found herself at war with Japan only.
Hitler's declaration of war on the USA made for reasons of short-term military gain, was, IMHO the actual turning-point of WWII.
AndyG