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.
After secretly crossing the Yalu River on 19 October, the PVA 13th Army Group launched the first phase offensive on 25 October, attacking the advancing UN forces near the Sino-Korean border.