As opposed to your southron myth-machine? The Supreme Court ruled that unilateral secession as practiced by the southern states was unconstitutional. The decision was Texas v White (74 US 700) issued in 1869.
When the Southern states seceded, there were neither words in the US constitution forbidding or permitting secession.
True. Secession may be described as an implied power. But there are many implied powers, and one of them is the power granted Congress to approve the change in the status of a state. That would include leaving.
Read The South Was Right by Kennedy & Kennedy.
I've read several books by the Kennedy brothers. I've never been impressed.