The Korean War (in South Korean Hangul: 한국전쟁; Hanja: 韓國戰爭; RR: Hanguk Jeonjaeng, "Korean War"; in North Korean Hangul: 조국해방전쟁; Hanja: 祖國解放戰爭; MR: Choguk haebang chŏnjaeng, "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 support of the United Nations, with the principal support from the United States (US)). The war began on 25 June 1950 when North Korea invaded South Korea following a series of clashes along the border.
As a product of the Cold War between the Soviet Union and the United States, Korea had been split into two sovereign states in 1948 with the border set at the 38th parallel. A socialist state was established in the north under the communist leadership of Kim Il-sung and a capitalist state in the south under the anti-communist leadership of Syngman Rhee. Both governments of the two new Korean states claimed to be the sole legitimate government of all of Korea, and neither accepted the border as permanent. The conflict escalated into warfare when North Korean military (KPA) forces—supported by the Soviet Union and China—crossed the border and advanced south into South Korea on 25 June 1950. The United Nations Security Council authorized the formation of the United Nations Command and the dispatch of 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, the ill-equipped and underprepared South Korean Army (ROKA) and the US 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 KPA troops in southern Korea. Those who escaped envelopment and capture were forced back north. UN forces invaded North Korea in October 1950 and moved rapidly towards the Yalu River—the border with China—but on 19 October 1950, Chinese forces of the People's Volunteer Army (PVA) crossed the Yalu and entered the war. The surprise Chinese intervention triggered a retreat of UN forces back below the 38th Parallel by late December.
In these and subsequent battles, Seoul changed hands four times, and 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 the Korean Armistice Agreement was signed. The agreement created the Korean Demilitarized Zone (DMZ) to separate North and South Korea, and allowed the return of prisoners. However, no peace treaty was ever signed, and according to some sources the two Koreas are technically still at war, engaged in a frozen conflict. In April 2018, the leaders of North and South Korea met at the DMZ and agreed to work towards a treaty to formally end the Korean War.