There was never a provision in the Constitution for a State to leave it peacefully.
Look. We differ on how we read the same US Constitution. The fact is anything not specified as belonging to the Federal government or specifically prohibited by the Constitution is open to each state to determine as per the 10th Amendment. No doubt that my experiences as a career US Army Officer, directly descended from a defender of Ft. Moultrie in the Revolution whose grandson died in the Civil War defending South Carolina against an invasion from other states - and a father who was a B-17 crewman in WWII - I take the right of free people to self government very seriously. I refuse to buy into the idea that any agreement made by any group of sovereign states or nation is a decision that their posterity must blindly follow for decades or hundreds or thousands of years into the future. Individual self government is a natural right that is not to be taken lightly by any central government anywhere or anytime. Sic semper tyrannis.