Referred to many in Korea as simply 6-2-5 (육이오), in remembrance of when the communist North Korea’s invasion of South Korea began (June 25, 1950), and hence marking the beginning of this war (reported as the first military conflict of the Cold War), the Korean War (also referred to in the U.S. history as the “Forgotten War”) was one of the most deadly conflicts in world history, reportedly claiming a total of 2.5 million lives.
Following World War II, Japan’s colonization of the Korean peninsula ended. What followed was the communist Soviet Union and the United States dividing up control of Korea, with the 38th parallel north latitude serving as the boundary between their zones of control (not unlike with Germany after WWII). As the Cold War brewed between these two new superpowers, North Korea, aided principally by the Soviet Union and communist China, invaded South Korea. The South was, in turn, aided principally by the United States and the United Nations peace-keeping forces to push back the communists' attempted takeover of the entire Korean peninsula.
In July 1953, the two sides finally signed an armistice/truce and ceased fire, thereby bringing the military conflict to an end. However, to this day, the two nations have not signed an official peace treaty, and hence both nations are technically still at war. As part of this armistice, the Korean Demilitarized Zone (the “DMZ”) was created. The DMZ, which only generally follows the 38th parallel north latitude (since it does not neatly cross the 38th parallel but is at an angle moving upwards in a north-east direction) essentially still serves as the border between North and South Korea.
For more information on the Korean War, please also refer to the following weblinks from Encyclopædia Britannica and CNN.com:
The Korean War
(June 25, 1950 to July 27, 1953)