When was korea unified?

It was never truly unified and has stayed divided since 1945 with the communist north and democratic south. Before any of that it was united but not united from two separate entities as it was controlled by the Japanese and before that independent as a kingdom/empire of Korea, and before that part of China. In between the two can be debatable parts of history over ownership but this is just it simplified

The Division of Korea began at the end of World War II in 1945