From Wikipedia

Cold War Korea

The Korean War was an armed conflict between North Korea (Democratic People’s Republic of Korea, DPRK) and South Korea (Republic of Korea, ROK), which began on 25 June 1950 and paused with an armistice signed 27 July 1953.

The war was a consequence of both countries aggressively attempting Korean national reunification under their respective governments. The Korean peninsula was politically divided as a legacy of the geopolitics of defeating the Japanese Empire on the peninsula in 1945. Soviet forces fighting the Japanese advanced up to the 38th Parallel, which later became the political border between the two Koreas. Despite talks in the months preceding open warfare, continual cross-border skirmishes and raids at the 38th Parallel, and the political frustration of failed all-Korea elections in 1948, escalated to warfare. The reunification negotiations ceased when North Korea invaded South Korea on 25 June 1950.

The United States and the United Nations intervened on the side of the South. After a rapid UN counteroffensive reversing the initial North Korean invasion, the People's Republic of China (PRC) intervened on the side of the North. The fighting ended with an armistice that approximately restored the original border between the Koreas; it became the Korean Demilitarized Zone. North Korea unilaterally withdrew from the armistice on 27 May 2009.

During the war, both North and South Korea were sponsored by external powers, thus facilitating the war's metamorphosis from a simple civil war to a proxy war as a part of the larger Cold War.

As military science, the Korean War combined First and Second World War strategies and tactics—swift infantry attacks followed air bombing raids. The initial mobile campaign transitioned to trench warfare, lasting from January 1951 until the 1953 border stalemate and armistice.