Begun as a war between South Korea (Republic of Korea) and North Korea (Democratic People's Republic of Korea), after the North's invasion of the South, the conflict swiftly developed into a limited international war involving the U.S. and 19 other nations. From a general viewpoint, the Korean War was one of the by-products of the cold war, the global political and diplomatic struggle between the Communist and non-Communist systems following World War II. The motives behind North Korea's decision to attack South Korea, however, had as much to do with internal Korean politics north and south of the 38th parallel (the boundary between the two republics) as with the cold war. Contrary to the prevailing view at the time, North Korea apparently attacked South Korea without the knowledge of either the Soviet Union or the People's Republic of China. The Soviet Union, which expected a war at a later time, was boycotting the UN when the attack occurred. The Communist government of China, meanwhile, was hoping to invade the island of Taiwan without having to deal with a military response from the U.S.

Considerable civil strife south of the 38th parallel and growing opposition to South Korea's president, Syngman Rhee, persuaded the North Korean leader, Kim Il Sung, that he would be welcomed by many South Koreans as a liberator intent on overthrowing the Rhee government and reuniting the two Koreas. As a champion of Korean unification, Kim would also undermine ongoing opposition to his own regime in North Korea.

The war began on June 25 when the North Korean army, substantially equipped by the Soviet Union, crossed the 38th parallel and invaded South Korea. The U.S. immediately responded by sending supplies to Korea, and it quickly broadened its commitment in the conflict. On June 27 the UN Security Council, with the Soviet Union voluntarily absent, passed an U.S.-sponsored resolution calling for military sanctions against North Korea. Three days later, U.S. President Harry S. Truman ordered combat forces stationed in Japan deployed to Korea. American forces, those of South Korea, and, ultimately, combat contingents from Australia, Belgium, Luxembourg, Canada, Colombia, Ethiopia, France, Great Britain, Greece, the Netherlands, New Zealand, the Philippines, South Africa, Thailand, and Turkey, with medical units from Denmark, India, and Sweden, were placed under a unified UN command headed by the U.S. commander in chief in the Far East, Gen. Douglas MacArthur. The participating ground forces of these nations, the U.S., and South Korea was grouped in the U.S. Eighth Army. The action was unique because neither the UN nor its predecessor, the League of Nations, had ever used military measures to repel an aggressor.

Even after Truman committed American ground forces to Korea, the war continued to go badly. Before the North Koreans were stopped in August, they had captured Seoul, the capital of South Korea, and the Americans and South Koreans had been pushed back to a small perimeter around the southern port city of Pusan, extending about 129 km (about 80 mi.) from north to south and about 80 km (about 50 mi.) from east to west. American reinforcements were able to hold this small area, however, and on Sept. 15, 1950, Gen. MacArthur launched a brilliant amphibious invasion behind enemy lines, striking at the port city of Inch'on on South Korea's west coast, about 40 km (about 25 mi.) west of Seoul. In a coordinated move, UN forces broke out of the Pusan perimeter. Very quickly the North Koreans were routed and forced above the 38th parallel.

Sensing an opportunity not only to stop but also to roll back Communist expansion, President Truman approved orders for UN forces to cross the 38th parallel and push the enemy above the Yalu River, which separated North Korea from China. Despite repeated warnings from the Chinese that they would enter the war if the Americans came near the Yalu, UN forces crossed into North Korea on October 7 and later captured P'yongyang, its capital city. By October 25 some advance units had reached the Yalu; there they came into contact with Chinese volunteers who had moved into North Korea. After hard fighting in which MacArthur's units had to fall back, the Chinese retired and MacArthur continued his offensive.

Shortly thereafter, the Chinese struck again, this time in massive numbers. UN troops, overextended, outnumbered, and