Well, I would hope that we would become a bit more civilized in the past 800 years or so.
Citizens have rights. Might does not make right. If a collection of self-governing people choose to alter their form of government, that is entirely their right.
Of course, under a "might makes right" regime, the most oppressive and cruel dictator would be perfectly justified in his actions? If the United States government started imprisoning people without cause or suspended the "right" of free speech and replied, "well, we have all the tanks--what are you going to do about it?" you'd have no problem with that response? After all, might makes right....
You are assuming that the south was in the right. I believe that their actions were wrong. So from your point of view wrong should always triumph over right?