In '48 Reagan campaigned for Truman because Truman did a lot of things right --NATO, the Marshal Plan, and the Berlin Airlift, all of which halted the Russian empire at Greece, Turkey, and Austria.
Truman saw the need to stop Stalin and pushed for a huge expansion of the defense budget. The Taft Republicans fought the military every inch of the way saying that of course they supported the troops but they did not support the mission.
If we condemn Truman for the dumb things he did then we have to condemn Reagan too. If we can forgive Reagan by saying he changed with the '70's then we can forgive Truman by saying he would have changed had he lived that long. There's no having it both ways.
What made Truman such a Great President was not that he did not make mistakes, he did. It’s that he tried to do the right thing.
Sure, Truman got a few things right, including his decision to nuke Japan. But then there’s the matter of Alger Hiss, the loss of China, and failure to finish the job in Korea when we had the Commies on the run. One might also recall that Truman was a supporter of big government—remember the Fair Deal?—and big labor.