you beat me too it - China massed on the border preparing to cross over in support the NK. MacArthur wanted to bomb them Truman said no.
Wrong. McArthur pushed NK back to the Yalu river, to close for comfort for China who, the usual communist paranoia thought we would invade China after we finished NK. The Chinese swarmed across the Yalu, also sending mig 15s against our inferior P80s. McArthur ignored the Chinese and even denied they were in NK until it we were being pushed back down the peninsula after the fiasco at the Chosin reservoir. THEN, McArthur wanted to use Nukes, and rightly so, THAT is when Truman fired him.
There was more to it than that, McArthur had made Truman look foolish through out the Korean war and Truman's ego couldn't take it. He hated McA and, as most true Democrats, hated the military. Truman tried to disband the Marine corp, cut spending to the point where by 1950 our armed forces were unable to conduct proper training and as a result we got our asses handed to us at the start of the Korean War. McA pointed this out to the news media along with other things and kept doing so until McA finally fired him over the Nuke business.
Had it not been for McA and his brilliant plan to land at Inchon we would have been thrown back into the sea and Korea would now be completely communists. As it was after Truman fired McA the war went into a stalemate and things went to sh** and the BS cease fire that is still in effect, which led to Vietnam etc.
Truman was the beginning of the gradual eroding of the will to fight in our nation.