It started as a year of back-and-forth battles up and down the Korean Peninsula and continued for another two years as a bloody stalemate. The war pitted North Korean troops using Russian equipment and later aided by Chinese soldiers against a United Nations force of South Koreans, Americans and other nationalities.
Officially, the three-year conflict sparked when North Korean troops and tanks crossed the 38th parallel into South Korea on the morning of June 25, 1950. It is not clear whether the North Koreans launched the assault unprovoked or South Korean forces started the fighting.
Also officially, the attack was a surprise, although North Koreans troops were observed massing at the border days before the attack, and although political tension in the Korean peninsula had been building for years.
Korea had fallen under Japanese control in 1910 and stayed under Tokyo's rule until Japan's surrender in 1945.
The U.S. planners agreed on the 38th parallel, and the Soviets accepted. On September 8, 1945, just six days after Japan's surrender, U.S. troops moved in to occupy Korea. While relations between the U.S. and U.S.S.R. on the Korean peninsula were at first cordial, the two powers were building radically different states on their respective sides of the 38th parallel.
In the south, where left-wing sentiments - especially for land reform - grew in the waning days of World War II, the U.S. backed Syngman Rhee. The right-wing leader faced a guerilla insurgency in 1948-49 whih American advisors helped his military suppress.
In the north, the Soviets backed Kim Il-sung, a communist who soon developed a close relationship with China. He even sent some North Korean troops to assist Maoist forces in the Chinese civil war.
When border skirmishes, started mainly by the south, began in May 1949, the bulk of Kim's troops were helping Mao, so all-out hostilities never broke out. U.S. occupation force pulled out in June, 1949.
In June, 1950, Kim's troops were back, and while his army was no larger than Rhee's, it had gained battle experience in China. With Soviet and Chinese tanks, fighters and bombers, the North had the edge.
All that changed after fighting broke out on June 25. North Korean troops routed the South's forces, driving them back relentlessly toward the capital Seoul, only 30 miles south of the parallel.
In hurried meetings at the White House, the Truman administration - which had been criticized by Republicans for taking a soft stance on communist gains in Asia - decided on an all-out response, not just in Korea but in all the areas of Asia treatened by communist insurgencies.
Truman ordered U.S. air and sea forces to cover South Korean troops, sent the Navy's Seventh Fleet to protect nationalist China (Formosa, now Taiwan), bolstered the ranks of U.S. troops in the Philippines, where a Maoist Huk rebellion was underway, and sped the shipment of weapons to French forces fighting the Viet Minh in Indochina.
The U.S. then sought approval for a U.N. task force to send troops to Korea. U.S. Ambassador Warren Austin said the North's invasion was "an attack on the United Nations itself." The U.N. security council approved the mission, but only because - inexplicably - the Soviet delegate, who could have vetoed the move, boycotted the meeting.
But even with a green light from the U.N., America soon learned its armed forces had deteriorated since World War II. From a high of 3,323,970 in 1943, the U.S. had drafted less than 10,000 troops in 1949. The first three waves of U.S. forces to South Korea failed to stem the communist tide, and eventually, the U.N. forces withdrew to the Pusan perimeter, a narrow line of defense at the edge of the Korean peninsula.
There they stayed until Sept. 15, 1950 and the landing at Inchon, the port of Seoul. Planned by U.S. Gen. Douglas MacArthur - commander of the U.N. forces - this amphibious operation allowed the U.N. forces to cut North Korean forces in half and recapture the capital by Sept. 26.
The U.N. forces kept driving in two prongs: one from Seoul and one from Pusan in the south. They reached the 38th parallel, where the war had started, on Oct. 1, capturing the North Korean capital, Pynongyang, on Oct. 19 and pushed North to the Yalu river, the dividing line between Korea and China.
But by this time, Red China had crossed the Yalu into Korea. On Nov. 25, the U.S. Eighth Army encountered Chinese troops and was beaten back. As 1950 ended and 1951 began, U.N,. forces gave up much of the territory the had won. On Jan. 4, communist forces again captured Seoul.
The U.S. press dubbed the defeat a disaster. "If this defeat were allowed to stand," said Time magazine, "it would mean the loss of Asia to communism."
But then another push north by U.N. forces began, capturing Seoul on March 18. Less than a month later, President Harry Truman removed McArthur from command, citing insubordination.
After a series of north-south jousts between the sides, the U.N. troops solidified their line at the 38th parallel - which they had lost, then retaken, then lost again - in June, 1951. The Soviets then proposed peace talks, and negotiations began in Kaesong, broke down in September and restarted in August in Ponmunjon.
Meanwhile the fighting continued. One of the more infamous battles of the war, the fight for Heartbreak Ridge, ran from Sept. 5 to 23, 1951.
As talks dragged on a major point of contention was what to do with prisoners of war held by either side who did not want to return home. The U.N. refused to allow forced repatriation, a stance that prolonged the war significantly. Time reported in August, 1953 that 7,000 U.N. soldiers died between the time the U.N. took that stand and the end of the war.
In March and April, 1953, the battled continued, including the famed fight for Pork Chop Hill.
Finally, on July 27, North Korean and U.N. forces signed a cease-fire establishing a buffer zone between North and South Korea at the 38th parallel, pretty much where the war began. The two sides decided to let a commission of five nations, headed by India, handle the repatriation of POWs.
Transfer of POWs began in September, but controversy soon broke out over how the commission dealt with POWs who did not want to return home. The U.S. accused India of taking the communists side. The focus of the fury was a group of 22 Americans, held by the North, who opted not to come home.
Roughly 22,000 communist troops held by the South refused to go home.
The war killed at least 3.5 million people, the dead consisting of 2 million civilians, 1.5 million communist troops, and 36,914 Americans.