Would you allow the Texas governor to enter into an agreement with the Mexican government to control some of the excesses and smooth things over?
Suppose the Texas government tells Mexico to take a flying leap and starts shooting suspected Mexican drug couriers on sight, just as they appraoch to cross the border? If Mexico retaliates militarily to protect its citizens, what then? A federal fight or a state fight?
Well I dont think Texas can formally enter into a treaty Constitutionally but they can play nice if they want. And it would be a federal fight for the latter.