Well, a bunch of dudes in black robes probably went "Blah blah blah" in a way that was probably mostly little more than a post-hoc defence of the Civil War's winner.I'm fairly sure that after the civil war we made it illegal to succeed.
Would the USA retake it, though?But, even if its not explicitly illegal, you have to look at things logically. If a state succeeds then the US can just come in and take it back, and it would. You might have a few pissed off idiots in the sticks but any government body that had a hand in succession is getting taken care of pretty damn quick unless they have nukes.
A unilateral decision to secede suggests a severe breakdown of relations. Would the USA really be prepared to bloody-handedly coerce a resentful state into subservience - the deaths, the cost, the repression? It's not 1860 anymore, and the USA is not Russia or China.