I'm just trying to jog my memory here, but didn't we fight a war about that once? Maybe like back in the 1860s or something? Didn't the side asserting the right to secede from the Union lose?

And didn't the U.S. Supreme Court later rule (in a Texas case) that the union is "indestructible?"

Isn't Texas fairly big on forcing students in school to stand up and recite an oath of allegiance to the United States, an oath that makes reference to that Texas case, calling the union "indivisible?"

Nah, I'm probably misremembering. I'd look it all up in a high school history textbook, but I hear Texas wants to rewrite them ...