It all depends on how the US acts in Mexico, from there one of a few different outcomes may occur
What follows are some ideas in order of stability not anything else.
Probabilities and Stability are rated 1-10... sort of
1. US is perfectly understanding of Political and Social situation in Mexico and provides an only slightly limited democracy, while enacting quiet social reforms in the background. All states must meet certain adequate benchmarks to reach statehood.
Possibility: -11
Stability:10
2. US largely acts like a child with a new toy, but over all manages to keep most social groups happy, by doing the bare minimum to ensure health, and education, does not affect most Mexican states overmuch for the first 20 years or so.
Possibility: 2
Stability: 8
3. US is just enough of a racist ass, and makes the right deals with the influential and rich, in order to keep the rest in line. The rich get richer and the poor, poorer.
Possibility: 5
Stability: 7
4. US is a hypocritical racist. Pretends to actually promote democracy in Mexico, in reality it all becomes a gigantic playground for the rich, and a trash heap for every one else. The beautiful beaches, and tourist zones are kept pristine, everything else becomes a gigantic gutter.
Possiblity: 6
Stability: 5
5. US is too open and learing in its racism towards all Mexicans. Makes most Mexicans believe that dying with rifle in hand is better than living with Gringos up their A**es.
If the US decides to stay anyways, expect a typical bloody Guerrila warfare campaign, but with more religious fervor coming in from the part of the Mexicans. Constant Military presence needed in most areas where Guerrila warfare is possible.
After about fourty years or so either Mexico's spirit is broken, or the US has had enough... which do you think is more likely considering the American Public.
Possibility: 6
Stability: 2
There are probably more, but this is where my list ends.