<Li> Korean Armistice Agreement </Li> <P> The Korean War (in South Korean Hangul: 한국 전쟁; Hanja: 韓國 戰爭; RR: Hanguk Jeonjaeng, "Korean War"; in North Korean Chosŏn'gŭl: 조국 해방 전쟁; Hancha: 祖國 解放 戰爭; MR: Choguk haebang chǒnjaeng, "Fatherland Liberation War"; 25 June 1950--27 July 1953) was a war between North Korea (with the support of China and the Soviet Union) and South Korea (with the principal support of the United States). The war began on 25 June 1950 when North Korea invaded South Korea following a series of clashes along the border . The United Nations, with the United States as the principal force, came to the aid of South Korea . China came to the aid of North Korea, and the Soviet Union also gave some assistance to the North . </P> <P> Korea was ruled by Imperial Japan from 1910 until the closing days of World War II . In August 1945, the Soviet Union declared war on Imperial Japan, as a result of an agreement with the United States, and liberated Korea north of the 38th parallel . U.S. forces subsequently moved into the south . By 1948, as a product of the Cold War between the Soviet Union and the United States, Korea was split into two regions, with separate governments . Both claimed to be the legitimate government of all of Korea, and neither accepted the border as permanent . The conflict escalated into open warfare when North Korean forces--supported by the Soviet Union and China--moved into the south on 25 June 1950 . On 27 June, the United Nations Security Council authorized the formation and dispatch of UN forces to Korea to repel what was recognized as a North Korean invasion . Twenty - one countries of the United Nations eventually contributed to the UN force, with the United States providing 88% of the UN's military personnel . </P> <P> After the first two months of war, South Korean and U.S. forces rapidly dispatched to Korea were on the point of defeat, forced back to a small area in the south known as the Pusan Perimeter . In September 1950, an amphibious UN counter-offensive was launched at Incheon, and cut off many North Korean troops . Those who escaped envelopment and capture were forced back north . UN forces rapidly approached the Yalu River--the border with China--but in October 1950, mass Chinese forces crossed the Yalu and entered the war . The surprise Chinese intervention triggered a retreat of UN forces which continued until mid-1951 . </P>

When did the usa join the korean war