And it is likely that Someone—British or American—got the word to the Chinese that the US would NOT hit China if they intervened in Korea.
Brit spy Donald Maclean the source, to Russia, that US would not use atomic weapons, as MacArthur wanted to do if China got to the Yalu (”We should never fight a land war in Asia”- MacArthur). Mac was right-if Truman gave the OK, just the threat would have been these results. No communist N Korea, No Viet Nam