I like this idea. Below that we've got a Republic of Mexico that is everybody's whipping boy and would very much like some revenge against the yanquis.
An interesting idea for the story would be a gradual thawing of relations between Texas and California eventually leading to a reunion and recreating the United States, with the goal of crushing the FSA and reclaiming the north. Could that work far enough down the line? Maybe after a fierce North American War Texas and California recreate the USA but with more states rights so it's something like a more integrated EU?
In 1899, the Diaz dictatorship is engaged in a bitter civil war. In northern Chihuahua Uriel Carasco set himself up as a caudillo, taking on and defeating several Federalista armies. Armed insurgents periodically cross the border into Texas and Arizona. A raid by one of these bands results in the overrunning of a frontier Texas town and the killing of some 150 Texans, including women and children. Governor Richardson calls a special session of the Texas Congress to discuss how to deal with the situation. In the meantime, Rangers, bent on revenge,cross the border and bucher everyone in the small town of Carlota, just south of the Rio Grande