What about a state waging war or invading another state or country?

Maybe someone can help me out with this? I have libertarian friends who go on and on about both Pauls being peace lovers. I can’t find anything to help me answer this question when it comes to states. I would guess that the Pauls would be okay if Texas decided to fight with Mexico—because that’s a state, not the federal government. Anyone have solid knowledge of that being discussed by either of them?

Thanks.