When China lashed out : vimarsana.com

On the wintry night of 27 November 1950, Chinese troops suddenly descended upon the US 1st Marine Division and the 31st Regimental Combat Team around the frozen Chosin Reservoir, less than 100 kilometres away from the China-Korea border. Having failed to dissuade the United States with words from crossing the 38th Parallel and crushing its communist neighbour, China had started dispatching troops into North Korea in October. But it was only then that the military and indeed geopolitical implications materialised.
Surrounded and vastly outnumbered, the US forces at Chosin Reservoir managed to break out after two weeks of brutal fighting. But the war had turned decisively against the allies fighting in Korea under a United Nations flag. In addition to pursuing sanctions against China, the Truman administration publicly and privately mused about using the atomic bomb to stem the Chinese offensive, prompting a disconcerted British prime minister to make an emergency visit to Washington.