There was no surviving central Mexican government to deal with when Texas led the first of the incursions to deal with the Mexican drug cartels. Oh, there was a Mexican government. It was staffed with politicians picked by the Drug Lords, and it made the North Korean government of the time look functional. Texas had long since stopped recognizing it as a legitimate government, and when they moved in they did not seek verbal deals with that government. The Texans preferred their deals inked in lead, and they liked to filibuster. The locals that had been ruled for decades were honestly happy with anybody who freed them from Drug Lord rule. Even if they had to be arrogant Americans.