The Korean War was a military struggle fought on the Korean Peninsula from June 1950 to July 1953. 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 United States 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 United Nations (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 United States.
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 United States 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 a 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, General Douglas MacArthur. The participating ground forces of these nations, the United States, and South Korea were 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.