On June 25, 1950 the armed forces of Communist North Korea drove across the 38th Parallel that had divided the two Koreas since the end of World War II. The South Korean military, smaller and less well-equipped, was unable to stop the well-trained northern forces, and beat a hasty retreat.
Donald Gregg, a former U.S. Ambassador to South Korea and now president of the Korea Society in New York, explains that the invasion was based in part on a miscalculation by Josef Stalin and Mao Tse-tung, the leaders of the Soviet Union and China, which were North Korea's primary supporters. "They both guessed wrong and thought the United States would not intervene and so the decision was made to arm and support North Korea by the Soviets and so the attack was launched," he said.
However, U.S. President Harry S. Truman obtained a United Nations Security Council resolution authorizing a military response to the attack, and the United States, Britain and other nations were quick to send troops. Mr. Truman dispatched an entire army.
On September 15, General Douglas MacArthur, commander of the U.N. forces, launched a counter-assault that took the North Koreans by surprise. His troops recaptured Seoul, and soon after that the North indicated it would be willing to accept restoration of the 38th parallel as the two nations' dividing line.
MacArthur, however, felt that the two nations could be forcibly reunited under Seoul's control and the fighting continued. He pushed northward, and by November had virtually all of North Korea under his control. But 300,000 Chinese troops, which had secretly crossed the Chinese-North Korean border, quickly pushed the U.N. forces back again.
Former Ambassador Gregg says the Chinese advance exacted a heavy toll on the allies: about 40,000 U.N. troops died on the battlefield. There were 900,000 Chinese and one half million North Korean soldiers wounded or killed. Despite this it was really Stalin, he says, who was calling the shots.
"The Chinese called for a halt in the war, and Stalin, in a brutal message, said no," he said. "It was Stalin's death in early 1953 that really set the stage for the signing of the armistice. A lot of people have forgotten what a fight to the death the war was at that time, and Korea was the cockpit of that war."
The origins of the war are anchored in the Korean Peninsula's history. For centuries, China, Japan and Russia jockeyed for its control. Early in the 20th Century, Japan colonized the peninsula, and held it until being defeated in World War II.
In 1945, the Soviet Union, which had declared war on Japan at the very end of World War II and had sent troops into Korea, accepted the surrender of Japanese units above the 38th parallel. The United States took the surrender of the Japanese south of that line.
Within a few years, Soviet and U.S. occupation forces pulled out, leaving a Communist government in the North and a democratically-elected government in the South. Kim Il-Sung, the North Korean leader, decided to reunify the peninsula by force of arms, and with Stalin's blessing, ordered his troops to attack.
When the fighting stopped in July 1953, both sides had been devastated and the high hopes that came with the end of the 35-year Japanese occupation had been dashed. Peace had been brought about not by a formal treaty, but only by a fragile armistice, which remains in effect to this day. The ideological differences that sparked the war are still in place despite the end of the Cold War elsewhere in the world.
The United States is still a close ally of the South, with 37,000 U.S. troops based there to deter the North from another attack and to help defend South Korea if an attack does come. The stability produced by the U.S. presence has allowed the South to develop a thriving market-based economy.
North Korea by contrast remains a highly reclusive Stalinist state with few friends. Economic mismanagement and a series of natural disasters have brought the nation to the brink of economic collapse.
"There still is potential for conflict," said Derek Mitchell, a senior fellow at the Center for Strategic and International Studies in Washington. "What we also see as a legacy of that war is a pretty developed and economically vibrant East Asia as a result of that and, on the contrary, the situation in North Korea is a disaster. So it is probably the perfect example of dichotomy between vibrant capitalism and freedom, and a closed society, that is the bottom line of the Cold War."
Global concerns over North Korea's intentions have arisen again since October, when the United States said Pyongyang had admitted it was trying to enrich uranium to develop nuclear arms. Since then, the North has engaged in a series of provocative acts, including restarting banned nuclear facilities, and now says it already possesses nuclear weapons.
South Korea's new president, Roh Moo-hyun, has promised to maintain his predecessor's policy of engaging the North with joint projects and economic assistance. But even he has called the North's weapons program "intolerable," and South Korea - with some reluctance - is once again part of an emerging alliance arrayed against the North.