China: (Chinese sources): 183,108 dead 383,500 wounded 450,000 hospitalized 25,621 missing (Those who defected or were captured were included in missing) 7,110 captured 14,190 defected (U.S. estimates): 400,000+ dead 486,000 wounded
As a product of the Cold War between the Soviet Union and the United States, Korea had been split into two sovereign states. Both governments claimed to be the sole 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.
In these reversals of fortune, 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 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 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 demilitarized zone and agreed to sign a treaty by the end of the year to formally end the Korean War.
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.
In the U.S., the war was initially described by President Harry S. Truman as a "police action" as the United States never formally declared war on its opponents and the operation was conducted under the auspices of the United Nations. It has been referred to in the English-speaking world as "The Forgotten War" or "The Unknown War" because of the lack of public attention it received both during and after the war, and in relation to the global scale of World War II, which preceded it, and the subsequent angst of the Vietnam War, which succeeded it.