Let's assume for the moment that Texas is the only state that wants to leave. Now you have two options:
a) Use military force, kill thousands of people, and force the rest to remain in the union, lest they too want to be killed.
b) Let Texas go. This means Texas is free to join any other state on the planet.
The point is that the union (in any form, anywhere) exists only on voluntary grounds. Once a large enough territory wants to leave, you have lost them - if not physically then morally. Keeping people against their will is both dangerous and impractical. Even a short and victorious war against TX will leave too many scars.
I believe this is one of the reasons why Ukraine just voices their unhappiness, but makes no military moves to keep Crimea. It is simply not possible politically. We can argue about how many Crimeans exactly want to be part of Russia - 89% or 91% - but it's clear that Ukraine has no traction there. Ukraine was only acceptable when the state was more or less stable. Today Ukrainian Parliament raised taxes, and there is another crowd outside, trying to evict this set of rulers just weeks after they assumed control. This is exactly how anarchy works. Crimeans are probably happy that they escaped Ukraine just in time, even if you only consider taxes.
If option A is chosen, expect the 21st Century version of the Sherman option to be on the table, preceded by a northward relocation of weapons. Whatever is left, Texas will use to give a very good fight. Expect the deaths to be in the millions when all is done.
Option B is highly unlikely given that the departure would not go pleasantly. It would only happen if Option A renders Texas uninhabitable.