The Korean War was a war between North Korea and South Korea. 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.
Korea was 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.