As I've pointed out before, I think even on this thread, the only thing that was settled in 1865 was whether the 1861 secession was legal.
Nothing else was settled. What would happen if Texas, for instance, declared itself independent? Short of an act of war, there really isn't the US could do to stop it, and once there is a war, the question of the legality of secession is then reopened.
How would that be accomplished? Do you hold a referendum and find out how many want independence and how many don't? What would be the cut-off - would it be a simple majority or a super majority? What if it was regionalized, for example if east Texas voted overwhelmingly to stay but west Texas voted overwhelmingly to go. Would you split the state and leave with only the western part? And what about the federal property in the state, the national debt, social security and medicare, how do you settle those?
Only a fool would assume, even for a second, that the Federal Government would hesitate to force Texas back into the Union at bayonet point.