Nobody believed a dissolved Yugoslavia would be at war either. Ethnic cleansing followed. People don't realize how easy things can snowball, once you let in darkness.
Scotland was also "always divided". EU countries are still so divided in practice (this vote being proof).
Look at Gibraltar. Channel tax havens. Nuclear submarines and oil platforms in Scotland. All these situations will now have to be renegotiated. Negotiations between states sometimes fail, and when they do, there's always that extension of diplomacy behind the corner: war.
There are endless fault lines all around us, which will be opened by renouncing the superior structures keeping them together.
Watch this movie, it's brilliant: https://en.wikipedia.org/wiki/The_Second_Civil_War