The Korean War (in South Korean
RR: Hanguk Jeonjaeng, "Korean War"; in North Korean
MR: Choguk haebang chǒnjaeng, "Fatherland Liberation War"; 25 June 1950 – 27 July 1953)
 was a
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.
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 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.
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.
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.