Secession wasn’t lawful - especially the way the south chose to attempt it. A bloody war and a supreme court challenge proved that.
Says some anonymous guy on the Internet.
The question was settled at gunpoint, and a great number of the immediate descendants of the guys who actually wrote the Founding documents disagreed with the outcome.
Questions about the Natural Rights given to us by our Creator may also be settled at gunpoint, but that fact doesn't make the gunwielder any more correct about the Natural Rights in question.