U.S. Marines breakout from the Chosin Reservoir in North Korea.

MMM

The Battle of Chosin Reservoir took place about a month after the People's Republic of China entered the conflict and sent the People's Volunteer Army (PVA) 9th Army to infiltrate the north eastern part of North Korea.

MMM

On 27 November 1950, the Chinese force surprised the US X Corps commanded by Major General Edward Almond at the Chosin Reservoir area. A brutal 17-day battle in freezing weather soon followed. Between 27 November and 13 December, 30,000 United Nations Command troops (later nicknamed "The Chosin Few") under the field command of Major General Oliver P. Smith were encircled and attacked by about 120,000 Chinese troops under the command of Song Shilun, who had been ordered by Mao Zedong to destroy the UN forces. The UN forces were nevertheless able to break out of the encirclement and to make a fighting withdrawal to the port of Hungnam, inflicting heavy casualties on the Chinese. US Marine units were supported in their withdrawal by the US Army's Task Force Faith to their east, which suffered heavy casualties and the full brunt of the Chinese offensive. The retreat of the US Eighth Army from north west Korea in the aftermath of the Battle of the Ch'ongch'on River and the evacuation of the X Corps from the port of Hungnam in northeast Korea marked the complete withdrawal of UN troops from North Korea.

Source: Wikipedia