oh Hitler's reason for that had nothing to do with helping Japan it was this beautiful piece of "philosophy":

"A great power does not wait for war to be declared on it, a great power declares war"

and that's basically all that informed Hitler's decision to declare war on the largest economic power in the world lol

The US was defacto in a naval war with Germany for a few months by then, it was a matter of time before it became a shooting war so Germany more just made it official in a way that gained them a secured ally