The USSR would have taken over all of Europe, not just Eastern. What would have happened to the former British and French empires? Presumably they'd have been turned communist, but would their struggles for independence have gone the same way?

The Manhattan project would have been announced before it went anywhere, undermined during development and produced no real result. Japan would not have been nuked, but also would not have been invaded and the Pacific theatre would have in general developed in a completely different way. Without the American nuclear arsenal, the world may have skipped the cold war and nukes, or perhaps other states would have made them and used them in a heavy mutual bombardment without restraint.

Eventually in Europe, glasnost and perestroika would still have happened, and the USSR would still have fallen; but the geopolitical and economic importance of the USA would be little different to that of Brazil.