 The United Nations resumed the offensive on 24 November , shortly before the Chinese began their own Second Phase Offensive . The Chinese 13th Army Group launched a series of surprise attacks on the night of 25 November , pushing the US Eighth Army back to the Chongchon River . The Chinese inflicted heavy losses on the South Koreans and decimated the US 2nd Infantry Division on the right flank , as the US Eighth Army began a long retreat south . At the same time , the Chinese 9th Army Group ambushed the US X Corps near the Chosin Reservoir as the freezing winter weather set in . Although the US Eighth Army succeeded in avoiding encirclement , the US X Corps had to be evacuated by sea from Hungnam during December 1950 , despite inflicting heavy casualties on the Chinese . MacArthur 's forces were expelled from North Korea and withdrew to the 38th Parallel , where they sought to once again establish defensive positions . In reserve at the start of the renewed Chinese offensive , the 27th British Commonwealth Brigade was spared the initial brunt . Yet the offensive soon forced the UN forces into a disorganised withdrawal . 3 RAR withdrew 320 kilometres ( 200 mi ) in nine days , arriving at Uijeongbu , 24 kilometres ( 15 mi ) north @-@ east of Seoul , on 11 December 1950 . There the British and Australians occupied defensive positions in an attempt to secure the northern approaches to the South Korean capital during the Battle of Uijeongbu . 
