After nearly 40 years of Japanese rule, Korea formed a unified independent, democratically elected government with representation from right, center, and left political leaders on September 6, 1945. The very next day US forces showed up to dissolve the government and keep power in the hands of the Japanese.
The subsequent election in South Korea was boycotted by nearly every political group.
In the late 1940s, Kim Gu had unification talks with Kim il-Sung but the CIA had him assassinated in 1949.
From there it’s a long chain of military dictatorships, coups, and suppression of democratic uprisings, with US influence and support.