The Korean War (1950-1953) was a devastating conflict started when Kim Il Sung, an ally -- some would say puppet -- of the Soviet Union launched an invasion of the southern half of the peninsula. For the next three years, intense fighting pushed the boundaries of the conflict up and down the peninsula until a ceasefire formalized the partition of North and South Korea along the 38th Parallel.
Early in the conflict, U.S. and Republic of Korea (South Korea) forces were pushed almost into the sea, so formidable was the North Korean offensive and so ill-prepared was the United States. A large-scale amphibious landing near the port of Inchon by U.S. forces relieved the siege under which ROK and U.S. forces had been subjected. As U.S. forces pushed North Korean forces northward, the Chinese Communist government under Mao Tse-Tung warned the U.S. that, if its forces came too close to the Chinese border, China would response militarily. General Douglas MacArthur, commanding general of U.S. forces in the Pacific, underestimated the Chinese and pushed forward. The resulting Chinese People's Liberation Army push southward drove U.S. and ROK forces back below the 38th Parallel. From then on, until the ceasefire, the fighting turned into a deadly stalemate.
Since the 1953 ceasefire, or armistice, the Korean Peninsula has remained divided along the 38th Parallel, the most heavily guarded and tense border in the world. The United States has retained anywhere from 30,000 to 40,000 troops in the south over the succeeding decades to deter another invasion from the North.
Periodically, but with disturbing regularity, the North Korean government has carried out highl provocative, and sometimes deadly, actions against South Korean. These actions have included the dispatch of North Korean special forces through hidden tunnels under the Demilitarized Zone along the 38th Parallel and by submarines landing near the South Korean coast. These commando missions have targeted the South Korean presidential compound and other political and military targets in the South. Also, in October 1983, North Korean agents planted a bomb in Rangoon, Burma, that killed 21 people and wounded another 23. Many of the dead were visiting South Korean officials, the target of the bombing attack. In November 1987, North Korean agents planted a bomb in a Korean Airlines passenger jet that exploded over the Andaman Sea, killing 104 passengers.
In addition to its terrorist acts directed against South Korea, the North has operated a system of labor camps where prisoners, mostly North Korean citizens whose sole "crime" was failure to praise the country's leader with sufficient vigor or to utter a complaint about chronic food shortages, are subjected to extremely harsh conditions. It is common practice for entire families to be imprisoned under such conditions because of the alleged disloyalty of a single family member.
Finally, the North Korean government has pursued, and successfully developed, nuclear weapons that it regularly threatens to use against its neighbors, mainly the South, Japan, and U.S. military bases in the region (most of which are on Japanese territory).
In short, the 1950-1953 conflict was a war. The peninsula since can best be characterized as a highly inflammatory Cold War.