Well, one possibility, one espoused by Pat Buchanan (among others), is that the Nazis and the Communists would have wiped each other out, preventing a Cold War.
While the Commies and the Nazis were wailing on each other, we should have devoted FULL resources to attacking the Japanese; we would have finished them off much faster, IMHO.
Anything is possible. You think Stalin would have defeated Hitler without US support?
I think the Brits would have had a problem without US support.
Or do you think the negotiated agreement subsequent to their declaring war on us, would have allowed us to continue supporting Hitler's enemies?