<P> 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 . </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> <P> 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 . </P> <P> 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 . In April 2018, the leaders of North and South Korea met at the demilitarized zone and agreed to sign a treaty by the end of the year to formally end the Korean War . </P>

Where did most of the fighting take place in the korean war