edit: I am not an expert on it, but I think there was some agreement done when they achieved statehood that allows it under certain conditions. The texans here probably know about it in detail.
The Five States of Texas. No matter what Rick Perry says, Texas won't secede. ... What Texas could do, however, is divide itself into as many as five states, a privilege given to it as a unique condition of its annexation to the Union in 1845.
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