Bybon, here is the short version. Truman had allowed the military to fall into complete disarray after WWII. The military was under-funded, under-manned and had little espirit de corps. MacArthur was a glory hound who refused to believe intelligence reports that the Chinese were involved in any great numbers. The South Korean Army was a joke and led by imbeciles. There was little support among Congress, the American people or our allies for the war effort. What was not known then, but is now, is that Mao was in charge of the war from the beginning and Kim Il Sung was nothing more than a Chinese puppet. Mao was prepared to lose one million men and guessed correctly that the U. S. would not use nuclear weapons against the Chinese in North Korea or Manchuria. As Omar Bradley said early in the war, "This is the wrong war, in the wrong place, at the wrong time with the wrong enemy."
For an excellent work that is pretty up-to-date and gives a good overview of all aspects of the conflict, read John Toland's, "In Mortal Combat".