I believe that states which decide to leave the union should sue, not just leave and force another internal war. Seems to me as I am not bound by the debts of my forefathers nor am I bound for any of their law violations, I should not be bound by their stupidity in not having an opt out clause. Frankly our foreign policy has supported this notion that is people states could leave the Soviet Union.
The USC is totally silent on the issue of secession. It is not unconstitutional. Fact.
There is another, untried, alternative to leaving.
That option is purge.
For instance, if all of New England were purged, the union would be better and stronger