I really, really hope such a day doesn't come. @GsElevator put it well the other day, "If it weren't for America we'd all be eating Bratwurst for dinner"
I think you'll find, that although the US did have an influence in the later days of WW2 in Europe, most of the really bad stuff was over by the time they came in.
Not to say that that they didn't assist immensely, but the victory over Hitler wasn't decided by US troops.
The war in the Pacific was a completely different (and somewhat sad) story.
Victory over Hitler was definitely not decided by US troops.
What's less clear is whether it was decided by US _trucks_ and the other material aid the US sent to the USSR during the war (e.g. pretty much all of the aviation fuel the USSR ended up using, a large fraction of their munitions, 2-3x the number of trucks the USSR produced during the war, large numbers of locomotives, etc).
As always, it's hard to figure out counterfactuals (e.g. if Russia had to build the 700k trucks they used during the war instead of the 200k they actually built, how many of the ~100k tanks they built would have remained unbuilt)? But the US, and to a lesser extent the UK, was significantly involved in the logistics of the Eastern Front, even though they had no troops there.
Or perhaps a more likely scenario - most of in Western Europe wouldn't be eating anything for dinner as we would never have existed. "Reasoning":
- Without an Allied landing, the Soviets would probably have "liberated" all territories occupied by the Nazis
- The UK would probably still be free - but an even more important base for operations against the Soviets than it was in reality
- The US would have felt even more threatened by the Soviets than they did
- A nuclear "Sunday Punch" first strike against the Soviets and all of their occupied territories would have become even more likely (and SAC showed no discrimation in who they would bomb until well into the 60s - the original versions of the SIOP involved bombing everyone who was 'socialist' even if they weren't allies of the Soviets and there was only one plan).
- The UK would have been utterly destroyed by Soviet weapons - but that seems to have been a part of pretty much all Cold War scenarios... :-|
[NB The Soviets on the English Channel is part of the scenario for Charlie Stross's novella "Missile Gap"]
Yes, that could have happened. Or maybe the Soviets would have found trying to keep all of Western Europe except for the UK under their boot a lot harder than it looked like from the beginning and they would have ended up in an Afghanistan like situation a lot earlier.
The problem with alternate realities is that no matter how intellectually interesting thinking about them might be we'll never know because we only have this one.
So, bratwurst, borscht or nothing, it's all just fantasy.
One thing is for sure, and common to all those scenarios, in that world anything between the UK and mother Russia would have been in for one hell of a ride.
