And the Declaration of Independence specifically says they have a right to leave if they want to do so.
Additionally, New York, Virginia and Rhode Island all stipulate in their ratification statements that they have a right to leave if they see the central government as becoming oppressive.
Additionally, both Massachusetts and Connecticut asserted a right to secede during the Hartford convention in 1814.
So there is a *LOT* of evidence that secession was legal, and virtually no evidence that states could be forced to remain in the Union against their will.
I agree with all that. I guess the thinking is that it is somehow OK to kill hundreds of thousands of people to force states to stay in a union the same states voluntarily created in the first place, which doesn’t make a lot of sense to me.