Korean War | Combatants, Summary, Years, Map, Casualties, & Facts | Britannica (2025)

Korean War, conflict between the Democratic People’s Republic of Korea (North Korea) and the Republic of Korea (South Korea) in which at least 2.5 million persons lost their lives. The war reached international proportions in June 1950 when North Korea, supplied and advised by the Soviet Union, invaded the South. The United Nations, with the United States as the principal participant, joined the war on the side of the South Koreans, and the People’s Republic of China came to North Korea’s aid. After more than a million combat casualties had been suffered on both sides, the fighting ended in July 1953 with Korea still divided into two hostile states. Negotiations in 1954 produced no further agreement, and the front line has been accepted ever since as the de facto boundary between North and South Korea.

Revolution, division, and partisan warfare, 1945–50

The Korean War had its immediate origins in the collapse of the Japanese empire at the end of World War II in September 1945. Unlike China, Manchuria, and the former Western colonies seized by Japan in 1941–42, Korea, annexed to Japan since 1910, did not have a native government or a colonial regime waiting to return after hostilities ceased. Most claimants to power were harried exiles in China, Manchuria, Japan, the U.S.S.R., and the United States. They fell into two broad categories. The first was made up of committed Marxist revolutionaries who had fought the Japanese as part of the Chinese-dominated guerrilla armies in Manchuria and China. One of these exiles was a minor but successful guerrilla leader named Kim Il-sung, who had received some training in Russia and had been made a major in the Soviet army. The other Korean nationalist movement, no less revolutionary, drew its inspiration from the best of science, education, and industrialism in Europe, Japan, and America. These “ultranationalists” were split into rival factions, one of which centred on Syngman Rhee, educated in the United States and at one time the president of a dissident Korean Provisional Government in exile.

In their hurried effort to disarm the Japanese army and repatriate the Japanese population in Korea (estimated at 700,000), the United States and the Soviet Union agreed in August 1945 to divide the country for administrative purposes at the 38th parallel (latitude 38° N). At least from the American perspective, this geographic division was a temporary expedient; however, the Soviets began a short-lived reign of terror in northern Korea that quickly politicized the division by driving thousands of refugees south. The two sides could not agree on a formula that would produce a unified Korea, and in 1947 U.S. President Harry S. Truman persuaded the United Nations (UN) to assume responsibility for the country, though the U.S. military remained nominally in control of the South until 1948. Both the South Korean national police and the constabulary doubled in size, providing a southern security force of about 80,000 by 1947. In the meantime, Kim Il-sung strengthened his control over the Communist Party as well as the northern administrative structure and military forces. In 1948 the North Korean military and police numbered about 100,000, reinforced by a group of southern Korean guerrillas based at Haeju in western Korea.

The creation of an independent South Korea became UN policy in early 1948. Southern communists opposed this, and by autumn partisan warfare had engulfed parts of every Korean province below the 38th parallel. The fighting expanded into a limited border war between the South’s newly formed Republic of Korea Army (ROKA) and the North Korean border constabulary as well as the North’s Korean People’s Army (KPA). The North launched 10 cross-border guerrilla incursions in order to draw ROKA units away from their guerrilla-suppression campaign in the South.

In its larger purpose the partisan uprising failed: the Republic of Korea (ROK) was formed in August 1948, with Syngman Rhee as president. Nevertheless, almost 8,000 members of the South Korean security forces and at least 30,000 other Koreans lost their lives. Many of the victims were not security forces or armed guerrillas at all but simply people identified as “rightists” or “reds” by the belligerents. Small-scale atrocities became a way of life.

The partisan war also delayed the training of the South Korean army. In early 1950, American advisers judged that fewer than half of the ROKA’s infantry battalions were even marginally ready for war. U.S. military assistance consisted largely of surplus light weapons and supplies. Indeed, General Douglas MacArthur, commander of the United States’ Far East Command (FECOM), argued that his Eighth Army, consisting of four weak divisions in Japan, required more support than the Koreans.