South Korea split from North Korea after World War II, and the Korean War that began in June 1950 when the North invaded the South ultimately became the first proxy battle of the Cold War, pitting active forces from the United States against Communist forces (with the People’s Republic of China backing the North). Despite an armistice agreement in 1953, the two Koreas have formally been in a state of war ever since, and the de-militarized zone between the two marks one of the most heavily armed borders in the world.