Truman didn’t pull all US troops out of South Korea. There remained the forces of KMAG (Korean Military Advisory Group), but they were only there for training and advisory roles. In an actual combat role, their use was next to nil.
He did, however, refuse to sell South Korea heavy weapons, armor, artillery, and combat aircraft. He claimed that it would provoke a “South Korean Invasion of the North”. Hence, why ROKA forces did so poorly during the early stages of the war. All the training in the world does little when one has no weapons to fight with.
To be fair, MacArthur is at least partially at fault for the heavy number of early US casualties. The Eighth Army was a parade-ground army, untrained and underequipped. (See TF Smith). But, then again, it was Truman who had cut the defense budget after the Second World War.
And, I hate to say it, but I agree. We should have learned after the Civil War.
Yes. Pretty much what a number of Democrats say that they want us to do in Iraq.