Interesting fact everyone seems to be missing is the Air War factor.
With no US, equates to no massive bombings in Europe. Germany could not move massive hardware, lost important infrastructure, and it was just a pain in the arse for them. That fact alone stalled the German machine (it is also simply amazing that the Germans were still able to fight well enough despite that fact).
With no US, UK would have been isolated, and so Germany would have been able move troops to stall the Soviet counterattack after Stalingrad. There would have been a bloody stalemate in the end, but Germany still in control of Europe.
With no US, equates to no massive bombings in Europe. Germany could not move massive hardware, lost important infrastructure, and it was just a pain in the arse for them. That fact alone stalled the German machine (it is also simply amazing that the Germans were still able to fight well enough despite that fact).
With no US, UK would have been isolated, and so Germany would have been able move troops to stall the Soviet counterattack after Stalingrad. There would have been a bloody stalemate in the end, but Germany still in control of Europe.