Relations between them became increasingly strained, and on June 25, 1950, North Korean forces invaded South Korea. The United Nations quickly condemned the invasion as an act of aggression, demanded the withdrawal of North Korean troops from the South, and called upon its members to aid South Korea. On June 27, U.S. President Truman authorized the use of American land, sea, and air forces in Korea; a week later, the United Nations placed the forces of 15 other member nations under U.S. command, and Truman appointed Gen. Douglas MacArthur supreme commander.

In the first weeks of the conflict the North Korean forces met little resistance and advanced rapidly. By Sept. 10 they had driven the South Korean army and a small American force to the Busan (Pusan) area at the southeast tip of Korea. A counteroffensive began on Sept. 15, when UN forces made a daring landing at Incheon (Inchon) on the west coast. North Korean forces fell back and MacArthur received orders to pursue them into North Korea.

On Oct. 19, the North Korean capital of Pyongyang was captured; by Nov. 24, North Korean forces were driven by the 8th Army, under Gen. Walton Walker, and the X Corp, under Gen. Edward Almond, almost to the Yalu River, which marked the border of Communist China. As MacArthur prepared for a final offensive, the Chinese Communists joined with the North Koreans to launch (Nov. 26) a successful counterattack. The UN troops were forced back, and in Jan., 1951, the Communists again advanced into the South, recapturing Seoul, the South Korean capital.

After months of heavy fighting, the center of the conflict was returned to the 38th parallel, where it remained for the rest of the war. MacArthur, however, wished to mount another invasion of North Korea. When MacArthur persisted in publicly criticizing U.S. policy, Truman, on the recommendation of the Joint Chiefs of Staff removed (Apr. 10, 1951) him from command and installed Gen. Matthew B. Ridgway as commander in chief. Gen. James Van Fleet then took command of the 8th Army. Ridgway began (July 10, 1951) truce negotiations with the North Koreans and Chinese, while small unit actions, bitter but indecisive, continued. Gen. Van Fleet was denied permission to go on the offensive and end the meat grinder war.

The war's unpopularity played an important role in the presidential victory of Dwight D. Eisenhower, who had pledged to go to Korea to end the war. Negotiations broke down four different times, but after much difficulty and nuclear threats by Eisenhower, an armistice agreement was signed (July 27, 1953). Casualties in the war were heavy. U.S. losses were placed at over 54,000 dead and 103,000 wounded, while Chinese and Korean casualties were each at least 10 times as high. Korean forces on both sides executed many alleged civilian enemy sympathizers, especially in the early months of the war.