http://en.wikipedia.org/wiki/Korean_War#China_intervenes_.28October_.E2.80.93_December_1950.29
On 4 August 1950, with the PRC invasion of Taiwan aborted, Mao ZedongZhou Enlainational security, they would intervene against the UN Command in Korea.[33]:83 President Truman interpreted the communication as "a bald attempt to blackmail the UN", and dismissed it.[77] The Politburo authorized Chinese intervention in Korea on 2 October 1950, the day after the ROK Army crossed the 38th parallel.[78] reported to the Politburo that he would intervene in Korea when the People's Liberation Army's (PLA) Taiwan invasion force was reorganized into the PLA North East Frontier Force. On 20 August 1950, Premier informed the United Nations that "Korea is China's neighbor ... The Chinese people cannot but be concerned about a solution of the Korean question". Thus, via neutral-country diplomats, China warned that in safeguarding Chinese
national security, they would intervene against the UN Command in Korea.
Same reference, but you left out the preceeding section that proves my point: Here I'll fix it for you:
" On September 27, MacArthur received the top secret National Security Council Memorandum 81/1 from Truman reminding him that operations north of the 38th parallel were authorized only if "at the time of such operation there was no entry into North Korea by major Soviet or Chinese Communist forces, no announcements of intended entry, nor a threat to counter our operations militarily
" On September 30, Defense Secretary George Marshall sent an eyes-only message to MacArthur: "We want you to feel unhampered tactically and strategically to proceed north of the 38th parallel."
On 1 October 1950, the UN Command repelled the KPA northwards, past the 38th parallel; the ROK Army crossed after them, into North Korea.
Six days later, on 7 October, with UN authorization, the UN Command forces followed the ROK forces northwards. The X Corps landed at Wonsan (in southeastern North Korea) and Iwon (in northeastern North Korea), already captured by ROK forces.The Eighth United States Army and the ROK Army drove up western Korea and captured Pyongyang city, the North Korean capital, on 19 October 1950. At months end, UN forces held 135,000 KPA prisoners of war.
Taking advantage of the UN Command's strategic momentum against the communists, General MacArthur believed it necessary to extend the Korean War into China to destroy depots supplying the North Korean war effort. President Truman disagreed, and ordered caution at the Sino-Korean border. "
UN Forces were all the way up to Unsan before the Chinese forces did anything. We should have stopped after taking the Capital Pyongyang City, which took six entire days.