In Texas v. White (1868), the Supreme Court ruled that Texas had no right to secede:
When, therefore, Texas became one of the United States, she entered into an indissoluble relation. All the obligations of perpetual union, and all the guaranties of republican government in the Union, attached at once to the State. The act which consummated her admission into the Union was something more than a compact; it was the incorporation of a new member into the political body. And it was final. 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.Okay, going to the Supreme Court for history is not exactly going to the most trustworthy source. The Congressional joint resolution that annexed Texas has no mention of it. The Texas Annexation Treaty (1844) makes no mention of such a right. This treaty was not ratified by the U.S. Senate, so it isn't legally binding, but you would think that if this right to secession was understood at the time, it would have been mentioned in one of these important documents. There is a right to subdivide Texas into multiple states without Congressional approval--a rather remarkable right indeed--but no mention of secession.
Can anyone enlighten me where this supposed right to secession appears?