The communists had good intelligence on our side. They knew in advance Truman would not nuke them. If Truman had instead told his staff that he was just waiting for the chance to send China to hell, the war would have ended with a united free Korea.
Life Magazine, in about 1957, published an interview/story with a high-ranking Polish Intelligence Official/Soldier named Monat Pavel, in which he detailed the Soviet involvement in starting and guiding the North Korean invasion of South Korea. His defection was a great boon to the West in that he gave an insider’s view of what actually went on before the June, 1950 invasion.
I strongly recommend historians interested in this period to read this article.