mexico is divided mainly by the capital because of its administration, the north for its raw resources and the south for farming.  north mexico is the one bringing in most of the wealth due to larger flatlands to build factories on and the amount of raw resources to be found for manufacturing.  if you annex the north of mexico, you will also be annexing a grand portion of the gulf of mexico which is rich in oil.  you will start seeing more plans for this annexation in the next spring.  they'll first make a whole theatre of "mexico is really angry because of deportation and tarrifs!"; they will explode the dirty bomb and then blame it on cartel-controlled mexico.  the us has done a lot of this false flags to invade countries.
Mexico