re: Hard to say if the Sovs and the Brits could have done it without US.
If America had sat out the war in Europe, Britain might have held out, but soon Russia and Germany would have gone at each other like Kilkenny cats. At some point, one of them would have won.
...and eaten the rest of Europe, Britain included.
Whether Europe and most of Asia was ruled by Stalin or Hitler, the situation would have sucked.