I did some reading on Texas vs. White -- while the court did rule that secession as practiced by the CSA was not authorized by the Constitution, it did envision some sort of secession as being possible.
The court did allow some possibility of the divisibility of the Union in the following statement:
The union between Texas and the other States was as complete, as perpetual, and as indissoluble as the union between the original States. There was no place for reconsideration, or revocation, except through revolution, or through consent of the States. (emphasis mine).
So the state has a right to revolt and rebel as the CSA did, but it can't secede unless it wins the war, or unless the other states in the Union vote to let it go.
In Texas v White, a dissent was written which, while denying Texas remained a state after secession, claimed not that Texas had successfully seceded but that she had become a "conquered province". Precedent was cited that a state is defined as an entity with representation in the United States Congress. During the Civil War, Texas had lost that representation. Thus, her status had become more analogous to an Indian tribe than to a state.
Now, we can nitpick this to death, of course. A lot of good 19th century legal minds disagreed with the court's ruling, based on what I was able to find about it. The thing about this case, also, was that it wasn't brought to determine the legality of secession -- it was brought to sort out the issue of the sale of U.S. government bonds by the state of Texas AFTER it seceded. Like a lot o fthings in the world of constitutional studies, it is murky.
