Both sides planned to institute their own government system throughout the entire peninsula, by force if necessary, and openly rallied towards this goal long before the actual invasion--i.e., Kim Il-Sung and Syngman Rhee had identical and mutually exclusive plans. There were bloody purges and massacres on both sides in the years between wars, and one of the sore points between the US and the ROK--to this day--is that the South Koreans were starting border skirmishes and wanted the US to arm them (and then go away--another story). At any rate, many Korean nationalists will tell you otherwise, but the evidence clearly suggests there would have been a war to control the peninsula with or without the Potsdam Conference and the Cold War jockeying between the US, USSR, and PRC. It was a classic case of a power struggle developing from a vacuum. What makes it complicated is the other vested interests.

This (the competing designs on the whole of the former Choseon Dynasty land) is likely covered in any history of the peninsula, but Korea's Place in the Sun, by Cummings, and The Koreans, by Breen--they make for good reading, especially in tandem because, in my opinion, they have pronounced, contrary biases that balance one another.

To piggy back on this question. At the out break of the war what happened to those living in close proximity of the now border. I understand the brothers fighting brothers part but am more wondering about the upper and ruling classes of people.

That question could have been worded better but my brain is mush today.