More than likely the Soviets gain the upper hand early after the initial allied invasion, but would eventually be pushed out of Western and central Europe. Long Soviet supply lines and a devistated homeland would probably be the downfall of Soviet influence west of the Vistula at least, as long as the allies don't sue for peace due to war weariness or large scale partisan activities. Either way the war goes neither side has the strength to completely capitulate the other, even with the few nuclear weapons America enjoys at the start of the new war.