In response to the United Nations invasion of North Korea, the People’s Republic of China (PRC) formed the People’s Volunteer Army (PVA). Although this force was composed of troops from the People’s Liberation Army, it was not officially tied to the PRC in order to avoid a general war between China and the United States. The PVA crossed the Yalu River into Korea on 19 October 1950.in wikipedia
25 Oct 1950–8 Jan 1951 Chinese intervention in Korea▲
On 25 October 1950, the Chinese People’s Volunteer Army (PVA) launched the First Phase Campaign, attacking United Nations forces in North Korea before withdrawing. This was followed by the Second Phase Campaign on 25 November, in which the PVA drove the UN from much of North Korea, retaking Pyongyang in early December. The Third Phase Campaign began in late December, with the PVA crossing the 38th Parallel and recapturing Seoul on 4 January 1951.in wikipedia