I'm not sure one could say that the US would have necessarily have been an antagonist more than anyone else in Europe. Had there been successful revolutions in Europe, I'm guessing that a lot of the "socialist in one country" and "third period" theories would not have been proposed. It's likely that Stalin would not have come to power as the Bolshevik Right - on which he initially leaned - probably would not have had a lot of standing. Trotsky may not have been an option either, necessarily, as he'd be remembered as a military man in founding the Red Army. It's quite possible that there'd be no NEP and the soviet government would have quietly retreated to its pre-Civil War days, safe behind soviet Germany, Soviet France, and Soviet Spain and Britain.
The Chinese probably would have had their revolution considerably quicker and in the US - if we are to assume that things are spreading this quickly - even former militants like the Bonus Army would have seen a viable alternative in the soviet system. Robbed of its largest trading partners, the depression Stateside probably would have gotten worse and only one viable alternative would have been visible on the world stage.
I'm speaking optimistically, of course, as we're assuming Europe embraces the idea. In reality, even if Germany fell to communism, France and the UK probably would have intervened drawing the US in. They might not have won, however. Pre-Franco Spain had a healthy worker's movement, the Paresian citizens would have to be controlled, and Ireland had set up soviets and sent delegates to Russia before anyone had heard of the Spartikans.
So who the fuck knows?
If you're interested, CLR James conceptualizes many of the movements at the time as a single
World Revolution that's largely cut down by the right-ward turn of the Bolsheviks.
Alis Volat Propriis; Tiocfaidh ár lá; Proletarier Aller Länder, Vereinigt Euch!