Blame Lincoln. The South tried to secede, but the Union of course did not let them. Big mistake, if you ask me, as "holy" as the fighting of the Civil War is presented in this country's history. I think the Northern US would have been better off without the South, culturally and economically.
But what do I know, as most of my ancestors (except for a few natives) came here after the Civil War.
It's been years since I've had a history class, so I'll just speak hypothetically. If I were running a country at risk of being divided, I might go to war to prevent secession if the rebelling territories had resources I needed. Examples include cotton, rivers/ports/shipping lanes, farmland, etc.
But what do I know, as most of my ancestors (except for a few natives) came here after the Civil War.