system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the main reason for the Korea country separation?

Honestly, it’s really hard for me to understand how a thing like that happens these days..
gpt
It's an unresolved Cold War-era conflict.

After WW2, the possessions of Nazi Germany and Imperial Japan were basically split between the various allied nations, and in the case of Korea that mean the USSR-led communists got the North, and the US-led capitalists got the South. This obviously didn't last long before war started between the two sides, with the North invading and almost completely annexing the South. The UN intervened behind the US, and reversed the entire conflict, with the South almost completely annexing the North. At this point, Communist China intervened on behalf of the North. With the US unwilling to use the nuclear option and trying to limit direct engagements between US and Chinese forces (so as to prevent WW3), the communists gradually pushed the South and their allies back to (very roughly) the original pre-war border.

At that point, China and the US put together an armistice, essentially freezing the conflict in place. The border has remained that way ever since.