Its function was to halt the sudden and rapid advance of Soviet troops into Korea before they occupied the entire peninsula, an action which would place them in position to dominate Japan in the post-war era. Soviet forces stopped their advance as Washington had requested, but the dividing line proved anything but temporary.
As the wartime alliance of the United States and the Soviet Union disintegrated and the Cold War deepened, a Soviet-style regime was created north of the dividing line, and a Western-oriented regime south of the dividing line. Thus was the ancient and homogeneous nation of Korea divided in the mid-20th century for the first time since the unification of the country under the Shilla dynasty in the 7th century.
In June 1950, North Korea, with Soviet and Chinese approval, launched an invasion of South Korea in an effort to reunify the country under communist auspices and came close to taking the whole peninsula. With United Nations Security Council approval, the United States and other nations rushed to the defence of South Korea.
When US and South Korean troops routed the North late that year, almost taking the whole of the North right up to the Chinese border, Mao Zedong's forces intervened to save the North Koreans. The Soviet Union, while steering clear of sending troops, surreptitiously provided warplane pilots.
On the UN side, the great majority of troops were from South Korea or the United States, but 14 other nations sent fighting forces: (in order of the size of their contributions) Britain, Turkey, Canada, Australia, Thailand, France, Greece, New Zealand, the Netherlands, Colombia, Belgium, Ethiopia, Luxembourg and South Africa. Hospital or other non-combatant units came from Denmark, India, Italy, Norway and Sweden.
When the fighting stopped, close to 4 million people had been killed or wounded, about half of them civilians. Cities on both sides had been devastated.
After three years of war, the final battle lines intersected the 38th parallel and were very close to the original line. Under the terms of the armistice, each side pulled back two kilometres from their final positions to create a no-man's land, a narrow, uninhabited ribbon that snakes across the peninsula from west to east.
Unlike post-1945 Germany, virtually no radio or television broadcasts have been regularly heard on the other side; travel to the other side was has been a rarity; and cross-border mail has hardly ever been permitted. Yet on either side of the line are divided families, brothers and sisters, aunts and uncles, who are also deadly enemies struggling against one another for half a century.
For the rest of the world, the Korean War proved an historic turning point. It led the United States to shift decisively from post Second World War disarmament to rearmament to stop Soviet expansion. It cemented the Soviet alliance with China and firmly established the Cold War.
Don Oberdorfer The writer is author of 'The Two Koreas', pound;20, Little, Brown and Company