Political map of China, Japan, Korea, and Mongolia 24 January 1951 (Chinese intervention in Korea): By November 1950, the American-led United Nations counter-offensive in Korea (UN Offensive, 1950) had overrrun most of North Korea. Mao (Mao Zedong) responded by sending the newly formed Chinese People's Volunteer Army (People's Volunteer Army) across the border, forcing the surprised UN forces to retreat.