Good point Suave. Without the Brits, to be more specific without Winston Churchill, saying: "No, we won't negoiate a settlement..." American intervention would have been a non issue. Those dark days when England stood alone cannot be forgotten.
To be fair, it took a while for the US people to realize that this wasn't just another European pissing match like WW1, and to realize that the Facists were really as bad as they were. Even in Britain, there were those who somewhat admired facism (not Nazi racial theory per se) and would have been willing to settle if it weren't for Winston Curchill. Never has so much been owed by so many to just one individual.
Of course, the US was still training with wooden tanks at the time so there were some practical reasons to hesitate as well, and we knew that within a handful of years we would be fighting a major war in the Pacific. Pearl Harbor was the stiff dose of medecine that shifted things into high gear, for a country that, like all the others non agressors hesitated going to war for as long as possible.
And, the Russians at the cost of 22 million, made the job that much easier in the end. Given Germany's production and manpower limitations they were bound to lose once either the US or a revitalized USSR ramped up, but it took a team effort to make that happen.
Charon