The Seperatists within Mexico, created as a response to the centralization of Mexico, continue to be a thorn in the side for the government of Santa Anna. The Government of Santa Anna is still confident it can regain control of the rebel territories, including those within dispute with Texas. Texas is annexed by the United States, leading to war between Mexico and the US. Mexico loses badly. The seperatists, taking advantage of the situation, begin a mass offensive against the remnants of the Mexican Government. America, not wanting a mess like this to carry on, gathers delegates from the seperatists and Mexico. In the treaty that follows, with the US having a strong presence in negotiations, the following territorial exchanges are made. America annexes all Mexican territory north of the Rio Grande and all of Baja. The Republic of the Rio Grande annexes Chihuahua, Durango, Sonora, and Sinaloa. The Republic of Yucatan doesn't annex any more than it already had claims to.
(Did my best to have that map make sense. Colored it in at a mexican restaurant. Grey = USA. Red = Rio Grande. Orange = Mexico. Green = Yucatan. Ignore that Yellow... for now.)
Feel free to continue this alt-hist on your own, or wait for the "canon" continuation.