I think it would have been rather difficult for Roosevelt to declare war on the Germans. They were not active allies of Japan in the full sense of what that means. For instance, the Japanese were not helping the Germans fight Russia, nor were the Germans helping the Japanese fight in China or anything like that. They had a friendly association, not a binding military alliance.

I suspect it would have taken Roosevelt several months at least to persuade Congress to declare war on Germany, had not Hitler solved the problem for him by immediately declaring war on the USA.

Why Hitler did that is inexplicable, except that he acted out of sheer passion, devoid of any rational thought on the matter whatsoever. It was the dumbest thing he could have done under those circumstances.