Arguably, you could say one of the primary reasons Russia was able to successfully defend against the Germans was the fact that the Germans beat themselves, independant of the americans. Combine the ill-planned 'Operation Barbarossa' with a tenacious, motivated, well-led enemy, the Germans really had no chance.
Some factors of the German defeat:
1. Germany sent in a field army, not completely comfortable in urban environments
2. Germany was ill-prepared, both men and equipment for the cold environment
3. Germany was fighting a multi-front war (the only relationship to the americans)
4. Too long of supply lines, later succeptible to air attack (sturmavik)
In conclusion, I won't take anything away from the american involvement in WW2, they did their part, but to attribute any part of the German defeat in the Russian campaign to the americans, would be, in my opinion, disingenious to the Russian people who fought and died to save their homeland. Not to mention revisionism.