On October 1 the South Korean army moved into North Korea as MacArthur issued a demand for North Korea to surrender. North Korean leader Kim Il-Sung requested help from China and despite Zhou En-Lai’s warning of Chinese intervention should UN troops enter North Korea they did so beginning on October 7, with US and supporting forces driving up both coasts, separated by mountainous terrain. Within two weeks Pyongyang, the North Korean capital, was captured and UN troops were nearing the Chinese border. MacArthur was already lobbying to enter China and destroy supply depots north of the Yalu River.
Josef Stalin informed Chinese and North Korean leaders that the Soviet Union would not send troops to intervene in Korea, and suggested that the Chinese send several divisions into Korea. On October 19, 1950, 200,000 Chinese troops, designated by Mao as the People’s Volunteer Army (PVA) began entering Korea, undetected by US reconnaissance aircraft. Only four days earlier MacArthur met with Truman on Wake Island, assuring the President that the Chinese were unlikely to intervene and that if they did they would be easily defeated by the UN forces.
The first attack by the Chinese was against South Korean units in late October. On November 1 the Chinese troops attacked an American unit, the Eighth Cavalry Regiment, and sent it into retreat. The Chinese did not pursue the retreating Americans, instead withdrawing into hiding in the mountains. On November 24 the Americans launched what was believed to be the final assault of the war, attacking on both coasts. The Chinese were waiting in ambush. Chinese counterattacks drove the US Eighth Army into what became the longest retreat in the history of the United States Army.
That the Eighth Army was able to retreat at all was thanks to the rear guard action fought by Turkish troops, which bought the withdrawing UN forces time at the cost of heavy casualties among the Turks. On the eastern side of the Korean Peninsula US Marines and Army units were encircled by Chinese troops at the Chosin Reservior. For seventeen days in the bitter cold and snow, they executed a fighting retreat. It was so cold that batteries failed, medical supplies like plasma and morphine froze, and frostbite occurred on exposed skin quickly.
UN forces across North Korea withdrew in fighting retreats, and after establishing a defensive perimeter around the port city of Hungnam MacArthur ordered the troops evacuated. Nearly 200 ships of all types took part in removing the troops and bombarding the Chinese troops with Naval gunfire. After destroying the port facilities to deny their use to the Chinese and North Koreans, the last UN forces were evacuated on Christmas Eve, 1950. The Chinese occupied the city the next day. The UN forces had surrendered all of the gains made after crossing the 38th parallel, despite inflicting heavy casualties upon the enemy.