Abraham Lincoln dealt the most serious blow to the rights of the individual States in our Republic when he denied them the right to secede from teh union. He set us up for the Big Brother, big-government we have now in which Federal courts routinely over rule States' decisions.
Today the Confederate flag stands as a reminder of how the Constitution has been twisted by teh Federal government to suit its own goals, and how we are now a Constitutional Republic in name only. Of how we are now a Federal Government in which decisions are made by mob rule.
Slavery was an issue during teh Civil War, but it was never the issue. Why did so many free black people willingly fight for the Confederacy? Do you know how prevalent slave-owning was in teh North?
*Always beware whenever anyone offers you a "simple answer" for anything.*