I feel like Truman was trapped after public approval of his Fair Deal
went down. The Red Scare and the spread of McCarthyism would have
occured with or without Truman's attack on communist nations.
Relations with the Soviet Union were declining while FDR was president,
and communists were very available targets as the public became
refocused on national problems after WWII. Truman's Fair Deal ended up
alienating both conservative Southern Democrats (because of Truman's
emphasis on civil rights) and Republicans (who wanted to restrain the
government). An attack on national communism and on the spread of
communist nations was, to some extent, a way of uniting Americans and
reaffirming the American faith in democracy.
Communism had also led to the rise of dictators and the fascist
governments that plagued Europe during WWII. This added to American
apprehensions about the spread of communism, and justified a provactive
attack on communist governments for many Americans. Looking back
Truman does seem overly aggressive, but the American public (and the
government, which had experienced a frusterating stalemate in
negotiations with Stalin and was threatened by communism in East
Germany) was primed for a war on communism.