Yes, and I seem to recall that Texas, as a condition to join
The Union, required a clause giving them the specific right
to withdraw from The Union if they determined that it was within their best interests.
Here's the Treaty of Annexation. I don't see the clause you're referring to. Can you point me to the document that contains it?
Yes, I think you are correct. Their agreement to join the Union was not just an agreement but a treaty.
There are many lawyers here who could tell us about that agreement versus the agreement on the part of the earlier states and commonwealths.