Hmmm, well I think if the Feds actually were to let Texas go they'd first remove all their military personnel and hardware from the state, leaving it vulnerable to attack. With a 1200 mile border with Mexico and an even longer one with the United States i'd guess that half the population will be permanently engaged in defending them.
If the Feds don't want to let Texas go (the more likely scenario), then between the Armored division stationed at Ft Bliss, the Armored Corps stationed Fort Hood and the many other federal military bases that are located in the state you basically have a modern Shermans Army already in position to begin a new March to the Sea.
