Let's completely destabilize the country on our southern boarder. Might as well take out an ad in every South American newspaper: Insurgents Wanted to fight the United States! It shouldn't be too hard to tell the difference between Mexican peasants and Columbian Guerrillas.
With the Mexican army fighting us, the drug cartels could overthrow the Mexican government. Super, then the boarder states would have to deal with hundreds of thousands of refugees fleeing the new Narcokleptocracy.