The agreement of other states is no more required than the agreement of the other kingdoms of the British Union. We didn't need approval from Wales, Scotland, Ireland or England to exercise the right of self determination.
The Supreme Court ruled that in 1869.
The Supreme Court made that crap up because they were kowtowing to the political winds of the time. If you don't grasp how that works, I will point out Roberts, Kavanaugh and Coney Barret as examples of how that works in our current era.
The vast majority of actual real evidence demonstrates that a Union joined voluntarily could be left voluntarily.
To my knowledge, all you have to the contrary are two letters from Madison, and one of those written 40 years after the fact, and directly contradicted by the actual ratification statement of Virginia, and of which he was part of the group who wrote it.
We needed a war, which we one. The South tried the same solution, and they lost. I guess they just didn't want it as badly as the founders did?
The Supreme Court made that crap up because they were kowtowing to the political winds of the time.
When it comes to making crap up who better than you would know? Kind of like all those powers the vice president has to reject electoral votes that you made up out of whole cloth.