I enjoyed the video game a lot, but I have it on the Xbox game consule, so the plot may be different. The timeline is pretty good and if you stretch it, possibly plausible to a certain extent. Even without the United States, Hitler won't win. The Soviet Union and the United Kingdom can still fend off Germany. I would think at least a few nations formed from the old USA would join the allies also. If countries like the Industrial States of America, Hollywood and the Empire State join the allies, I see an Allied victory. Japan is a whole another story. They would take serious advantage of the situation in North America. I'm unclear what might happen.
Wouldn't the nations of North America have air superiorty with the planes they have? Since the roads in the United States were destroyed they had to depend on air travel and they made HUGE advancements in the 30s and 40s I believe. I have the Crimson Skies book right in front of me and the timeline is very cool. Other countries like Columbia (nations former capital) and the Dixie Confederation might also put up a fight against Germany. The book explains exactly how the USA dissolves. What happens in WW2 still seems uncertain. We can only speculate.