From https://metaculus.com/questions/15644/second-korean-war-by-2050/

Both the Republic of Korea and the Democratic People's Republic of Korea claim sovereignty over the entire Korean Peninsula. This dispute led to the infamous Korean War, during which North Korea invaded South Korea in order to unite the peninsula under their rule. This attempt failed, leading to a military stalemate and several million people dead. The war ended with a ceasefire and the established of the Korean Demilitarized Zone dividing the two nations. However, de jure they remain at war.

There have been many close brushes with a return to large-scale, de facto warfare. The closest was the Korean DMZ Conflict of the late 1960s, a series of low-level military clashes that left several hundred dead.