There was a point in Korean War when the US and SK pushed all the way to the Chinese border. Mao hit back with a vengeance. After that the US military were unwilling to take the war north of the 38th parallel.
And this one was General MacArthur biggest blunder, ignoring the civil authorities and acting like he was really an Emperor. WW3 was almost started; after the ensuing stalemate everyone kind of chickened out. There are no easy solutions, used to be that we will quiet NK with food and money; now they are really dangerous.