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1950: Aug. 1950: Nov. 1951: Jan. 1951: July

In 1949, the Republic of Korea was established in U.S.-occupied territory south of the 38th Parallel, while the Democratic People's Republic of Korea was established in Soviet-occupied territory to the north. North Korean army built up its armed forces with weapons provided by the Soviet Union, while the South Korean army remained pitifully undermanned and underequipped.

On June 25 the North Korean army attacked across the 38th Parallel, and took Seoul within a week. President Truman ordered U.S. troops in Japan to help the South Korean army, which was rapidly retreating. The United Nations Security Council authorized military action to repel the invaders. Gen. MacArthur, who was in charge of the U.S. occupation forces in Japan, becomes head of the combined U.N. forces. By August the Americans and South Koreans establish a defensive perimeter in the southeast, as more reinforcements arrive at the port of Pusan.

On September 15, the U.S. 1st Marine Division landed at Inchon, and liberated Seoul (25 miles away) just a few days later. This cut off North Korean forces from their supply sources, forcing them to quickly retreat from their advanced positions near Pusan. After linking up with U.N. forces advancing from the Pusan perimeter and clearing South Korea of enemy forces in late September, the U.S. advanced across the 38th Parallel into North Korea. On October 20 they captured the North Korean capital of Pyongyang, and then rapidly moved north as far as the Yalu River, which marked the border with China. Amid reports that Chinese troops were moving into North Korea, however, these forces withdrew in early November. On October 29 the U.S. 7th Infantry Division landed at Iwon in North Korean, and soon traversed mountain roads to Hyesanjin on the Chinese border, while South Korean units advanced along the coast toward the northeast.

On November 24, the Chinese People's Liberation Army began an all-out offensive, quickly surrounding the U.S. 1st Marine Division at the Chosin Reservoir. U.S. forces and allied U.N. forces were forced to retreat, and the Marines at Chosin barely managed to escape from their entrapment, in the midst of freezing, snowy conditions. By early December the Chinese had retaken Pyongyang, and by mid-December they had crossed the 38th Parallel and captured the city of Kaesong. Isolated U.N. forces at Wonsan evacuated by sea on December 10, and those at Hungnam did likewise on December 24. The Chinese captured Seoul in early January, and continued south, while the U.N. forces desperately reestablished a solid front with the U.S. forces that had been evacuated in December.

In February U.N. forces began an offensive, backed by strong air support, and liberated Seoul (for a second time) by March. They crossed the 38th Parallel a second time, and advanced 20-30 miles beyond it in the center and eastern portions of the front, while the Communist forces retained control of Kaesong and nearby areas on the western side of the front. The pace of the war gradually slowed down, as the front barely moved during the two years of peace talks that followed. In July 1953 an armistice agreement was reached, and both sides pulled back from the front line to establish a 3-mile wide De-Militarized Zone (DMZ).

This line has remained the de facto border between North and South Korea ever since, but there was never a definitive resolution of the conflict. In the decades that followed, North Korea staged provocative attacks on numerous occasions, including the seizure of the U.S.S. Pueblo in 1968. During the early 21st Century, North Korea developed its own atomic bomb, and built up a considerable arsenal of such weapons. War could resume at any time.