TheLastBoyScout
Member
Germany declared war on us out of a sense of solidarity with their Axis ally Japan. This was not necessary; after all, Japan didn't give Russia a "second front" when he invaded.
Admittedly, FDR wanted to smash Hitler more than he wanted to beat Japan, but without Hitler's declaration of war, it would have been hard to get a declaration against Germany; if such a declaration came at all, it would likely have come too late (after we beat Japan, giving Hitler enough time to finish off Russia, and possibly overrun Britain).
Even if the British held on long enough for us to deal with Japan and ride to the rescue, I wouldn't put great odds on an invasion of Western Europe without the Eastern front to serve as a sink for German resources.
Admittedly, FDR wanted to smash Hitler more than he wanted to beat Japan, but without Hitler's declaration of war, it would have been hard to get a declaration against Germany; if such a declaration came at all, it would likely have come too late (after we beat Japan, giving Hitler enough time to finish off Russia, and possibly overrun Britain).
Even if the British held on long enough for us to deal with Japan and ride to the rescue, I wouldn't put great odds on an invasion of Western Europe without the Eastern front to serve as a sink for German resources.