The Korean War: Map of the 38th Parallel, the Front Lines and the DMZ

Korea was annexed by Japan in 1910 and ruled as a colony for 35 years. In August 1945, as the Soviet Union entered the war against Japan and its troops poured into Manchuria and northern Korea, two young American officers in Washington, Dean Rusk and Charles Bonesteel, were given half an hour to propose a line between the American and Soviet occupation zones. Using a National Geographic map, they chose the 38th parallel, which left Seoul in the American zone. Stalin accepted. About 16 million Koreans lived south of the line and 9 million north of it.

The occupation was meant to be temporary, but the two powers could not agree on a single Korean government. In the north the Soviets backed the communist guerrilla leader Kim Il Sung; in the south the Americans worked with the conservative nationalist Syngman Rhee. After UN-supervised elections held only in the south, the Republic of Korea was proclaimed in Seoul on 15 August 1948, and the Democratic People's Republic of Korea followed in Pyongyang on 9 September. Each claimed the whole peninsula. Use the 1944, 1946 and 1948 buttons to see the three stages.

By 1950 Soviet and American troops had left, and border clashes along the 38th parallel were common. With Stalin's approval and Mao's promise of support, North Korea's army, equipped with Soviet tanks, crossed the parallel at dawn on 25 June 1950. Seoul fell on 28 June. The UN Security Council, which the Soviet Union was boycotting, called on members to help South Korea, and President Truman sent American forces from Japan under General Douglas MacArthur. Sixteen countries eventually sent combat troops under the UN flag.

The North Koreans drove the South Korean and American troops back to a small corner of the south-east around the port of Pusan (Busan). Behind the Pusan Perimeter, a 230 km line along the Naktong River shown in blue on the map, they held on through August and early September 1950.

On 15 September 1950 MacArthur landed a US Marine and Army corps at Inchon, the port of Seoul, far behind the North Korean lines, despite tides of up to 11 metres. Seoul was retaken on 28 September and the North Korean army collapsed. Instead of stopping at the 38th parallel, UN forces crossed it on 1 October, took Pyongyang on 19 October and pushed on towards the Yalu River, China's border; the dashed blue line shows their farthest advance in November.

China had warned it would not tolerate American troops on its border. From October 1950 some 200,000 to 300,000 soldiers of the Chinese People's Volunteer Army crossed the Yalu in secret, and in late November they struck. Around the Chosin Reservoir, in temperatures of –30 °C, US Marines and soldiers fought their way out of encirclement to the coast, and about 105,000 troops and 98,000 civilian refugees were evacuated by sea from Hungnam in December. Chinese and North Korean forces retook Pyongyang, crossed the 38th parallel and captured Seoul again on 4 January 1951; the red dashed line marks how far south they reached.

Under General Matthew Ridgway the UN forces recovered, retook Seoul in March 1951, the fourth time it had changed hands, and pushed the front back to around the 38th parallel. When MacArthur publicly pressed to widen the war into China, Truman dismissed him on 11 April 1951.

From mid-1951 the front barely moved. Truce talks opened at Kaesong in July 1951 and moved to Panmunjom, but they dragged on for two years, mainly over whether prisoners of war who did not want to go home should be forced to return. Meanwhile the two sides fought bloody battles for hills with names such as Heartbreak Ridge, Pork Chop Hill and the Punchbowl, and American bombers destroyed almost every town in North Korea.

After Stalin's death in March 1953 and a new US president, Dwight Eisenhower, the talks moved forward. The armistice was signed at Panmunjom on 27 July 1953 by the UN Command, North Korea and China; South Korea's President Rhee refused to sign. It fixed a ceasefire line along the front, slightly north of the 38th parallel in the east and south of it in the west, with a Demilitarized Zone 4 km wide along it, the black line on the map.

About three million people died, the majority of them civilians, making it one of the deadliest wars of the Cold War. Military deaths numbered around a million: South Korea lost about 138,000 soldiers killed, the United States about 36,000, China perhaps 180,000 or more and North Korea several hundred thousand. Both sides committed mass killings of civilians suspected of supporting the enemy, and millions of Koreans became refugees, separating families who have never met again.

No peace treaty has ever been signed, so the two Koreas are technically still at war. The Demilitarized Zone, about 250 km long, remains one of the most heavily fortified borders on earth. South Korea later grew into a wealthy democracy, while North Korea became one of the world's most closed and repressive states, ruled by Kim Il Sung's son and grandson; the satellite photograph of the peninsula at night in the gallery shows the gap.