If it's pre-1949, I can see the United States being nearly killed on the ground before they, in desperation against the Soviet horde approaching the Rhine, detonate their atomic bombs over the Ruhr, causing immense damage and the crippling of the Soviet Army. The NATO armies in other states that are less hit by the Soviet offensive go to offensive mode and begin to push back.
HOWEVER... If the US were capable of holding the Soviets before they overran their lines (the Soviets had a huge ground advantage in the late '40s), then you might see the Americans deploying nuclear bombs to the Vistula to cut off the majority of the Soviet supply, or even launching atomic attacks on Russian cities from Turkey.
In any case, the USSR will ultimately lose a pre-1949 scenario.
Early-to-mid 1950's, after the USSR gets the bomb? I can see massive stalemating and death on both sides before a general ceasefire is called. The US would be the least scathed by the nuclear war, as the Soviet Union had no way to deliver long-range nuclear attacks before the ICBM. But the US is hampered by dealing with the irradiated ashfield that is Europe.
Late 1950's to 1960's? Complete destruction of the human civilization, no victors.