On 4 August 1950, with a planned invasion of Taiwan aborted due to the heavy US naval presence, Mao 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 Zhou Enlai informed the UN that "Korea is China's neighbor ... The Chinese people cannot but be concerned about a solution of the Korean question". Thus, through neutral-country diplomats, China warned that in safeguarding Chinese national security, they would intervene against the UN Command in Korea.
On 19 October 1950, Chinese forces of the People's Volunteer Army (PVA) crossed the Yalu and entered the war. The surprise Chinese intervention triggered a retreat of UN forces back below the 38th Parallel by late December.