With all due respect, 'we' didn't need to defeat Germany - England & Russia did. And if FDR wasn't illegally giving arms & aid to GB from the get-go (in violation of the Neutrality Act) our ships wouldn't have been attacked by the U-Boats and Raiders.
Which also brings up WHY Japan attacked us in the first place; FDR's asia policy was openly hostile to Japan as FDR pretty much declared Economic War them a long time before 1941. 'He' was choking them off from vital necessities for survival (like oil & rubber) and it guaranteed that they'd respond - and not with a nasty letter.
Anyway, after Dec 7th Germany was forced compelled to declare war on us do to their pact with Japan.
NOTE: I'm not justifying anything that Germany or Japan did (I still HATE detest the Japs). My point is that FDR was determined to get us into the war one way or another. And like a typical DemoncRAT -- the Law, Congress or 'the peoples will' be damned, he wanted us in and he got us in.