It’s really important for the allies that US is in the game. Russia can’t survive long unless Germany is threatened by US/UK to its west, and Japan can have free reign in the Pacific without the US to quell its expansion
My oldest son was both US and Russia and when Russia fell so did his spirits, especially when my uncle, the UK player, was not able to do anything against Germany or Japan. I struck UK and Russia early and often and it was too much for either one to handle. Next time we play I am taking the Allies so I can see if they will try what I did with Germany. They will be in for a surprise since I know what to do with the US and UK to help Russia and defeat Germany.