Korea wasn't always divided into North and South like it is now. While today the differences between the two seem as distinct as night and day, just 75 years ago they were essentially the same nation. Why then, is Korea divided into two countries?
The history of North and South Korea is complicated. The country was split in two after WWII, but the story really goes back to a time long before that. The history of the Korean Peninsula is a tale of a small kingdom being pushed around by much larger powers again and again. The split between the North and the South is just the latest manifestation of that phenomenon.