> State secession is not unconstitutional. <
And neither is it constitutional. It’s a damn shame the Founders were silent on this issue.
Side note: I believe that state secession should be allowed. It should be a long and involved process, taking perhaps a decade. But it should be allowed. However, the Constitution says nothing about this. So such matters are decided by the bayonet.
If it is not prohibited by the Constitution then there is no reason why it is not permissible. It's just a question of how you go about it. Unilateral secession as practiced by the Southern states wasn't the way.