Snake Featherston
Banned
Mexico would likely draw OTL's filibusters, only perhaps more so since now US support is pretty explicit. There is going to be some brutal attempts at ethnic cleansing and forest-clearing since the filibusters were all gaga for King Cotton.
The populous southern Mexican states will have to be given local autonomy pretty damn fast. I could actually see the US redrawing the borders to create fewer, larger states that gives power to the criollo elite into the State apparatus. The other trick--which the US did in the Philippines--is to create a local constabulary of natives to repress the other natives.
Actually, the Philippines might be a good comparison because--while geographically dissimilar--its population at the time of the US occupation was 7,409,000 and its a heavily Catholic non-white society.
The one thing about that, though, is that brutality didn't end the uprisings against Nazi Germany, even when the Nazi response was to kill anything human within yea far a mile radius in retaliation. 1840s America had enough problems coughing up the troops for this war, hiring filibusters to go play Bleeding Oaxaca isn't going to be anything but one of the key elements in the rapid degeneration of the United States into a multi-sided civil war.