A Supreme Court Case from 1868, Texas vs. White, also makes the idea of secession improbable. In that case, the justices determined that when Texas “became one of the United States, she entered into an indissoluble relation.”
The court’s decision means that no individual states are now allowed to secede.
“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,” according to the decision handed down by the justices.
Without consent from the U.S. or by means of revolution, Texas can’t secede from the union.