Wait a minute. I must modify my post #49 to you. The 10th Amendment: The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
One could fairly argue that since secession is not covered in the Constitution, the 10th Amendment gives that power to states.
Yes, I think you agree now. However whether the 10th existed or not states still can secede. On this we probably disagree.