Well, you forget, Texans themselves would not allow it and if push came to shove the country of Mexico wouldn’t stand a chance against Texas even if the federal government never did get involved!!!!!!!!!!!
I think New Mexico is the state most likely to move as a state. The demographic maps of Texas show a big difference between the big-bend and border areas, and the rest of Texas. So I would first expect county-by-county changes that lead to ethnic cleansing in souther Texas.
I think public schools offering instruction in Spanish with English as an elective secong language will be a powerful forcing mechanism to get the "Anglo" minorities to either leave or acclimate to Latino Dimmhi status.
That's where the Kosovo precedent would come in. NATO forces would come in to protect the oppressed Atzlaners. The UN would lay down sanctions and resolutions to condemn any attempt in Washington to help the racist white Texans harassing the embattled ethnic majority. Most of the pols in DC would already support the enlightened UN position and the rest would be too weak kneed to seriously oppose it.