What unilateral???
Each state, by the same means that they joined the union, could vote to leave. The Union should NEVER be allowed to vote on letting a state out -- it doesn't work that way and would be useless if it did.
With the exception of the original 13, each joined the Union only with the approval of the existing states through a majority vote in Congress. And I agree, leaving should have been done through the same means, with the approval of the other states through a majority vote in Congress.
The Union should NEVER be allowed to vote on letting a state out...
Why not? A vote is required to let them in.