The Korean War (in South Korean Hangul: 한국전쟁; Hanja: 韓國戰爭; RR: Hanguk Jeonjaeng, "Korean War"; in North Korean Chosŏn'gŭl: 조국해방전쟁; Hancha: 祖國解放戰爭; MR: Choguk haebang chǒnjaeng, "Fatherland Liberation War"; 25 June 1950 – 27 July 1953)[b] 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.
Korea was ruled by Imperial Japan from 1910 until the closing days of World War II. In August 1945, one day after the bombing of Nagasaki, the Soviet Union declared war on Imperial Japan, as a result of an agreement with the United States, and ended Japanese rule 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. 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 around 90% of the military personnel.
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.
After these reversals of fortune, which saw Seoul change hands four times, the last two years of fighting became a war of attrition, with the front line close to the 38th parallel. The war in the air, however, was never a stalemate. North Korea was subject to a massive bombing campaign. Jet fighters confronted each other in air-to-air combat for the first time in history, and Soviet pilots covertly flew in defense of their communist allies.
The fighting ended on 27 July 1953, when an armistice was signed. The agreement created the Korean Demilitarized Zone to separate North and South Korea, and allowed the return of prisoners. However, no peace treaty has been signed, and according to some sources the two Koreas are technically still at war, engaged in a frozen conflict.
As a war undeclared by all participants, the conflict helped bring the term "police action" into common use. It also led to the permanent alteration of the balance of power within the United Nations, where Resolution 377—passed in 1950 to allow a bypassing of the Security Council if that body could not reach an agreement—led to the General Assembly displacing the Security Council as the primary organ of the UN.