I tend to quickly tire of timelines that revolve around 'America kicking communist ass and saving the world 40 years early', but there's something strangely attractive about this one, since it pre-supposes the possibility that it's the Americans striking first.
There's something halfway plausible in that. A lot of literature in the US around the '40s and '50s revolved around the possibility of a triumphant war with the yanks marching into Moscow and being greeted as the best thing since Wonderbread. A lot of people I suspect were secretly hoping that the Russians would live up to the American's own propoganda and make some kind of gutless, cowardly surprise attack to give their boys a reason to kick ass.
The Soviets by contrast were effin' terrified of the prospect. Stalin's reason for maintaining the 'buffer zone' in Eastern Europe was to safeguard against another invasion from the west. As far as the Russians were concerned, the west was 'where the invaders came from'. Next to nothing suggests they contemplated striking first (except when they feared an imminent attack, obviously) and the more triumphalist literature of the time was hoping for victorious revolutions in the west, not victorious armies.
So, just entertaining the possibility, sometime between 1945 and 1949 we have a President who's even more paranoid about the Communists than Truman was (which is a pretty tall order...heck, just having Truman on one of his bad days would probably be sufficient) building up an invasion force and attacking the Soviet Union.
How about around the Berlin Airlift? The American leadership decides that the only way to save Berlin is to open the supply routes itself, and gives the go-ahead to an invasion plan, kicking off in 1948.
Things will go really badly, I reckon.