Mexico would be hard to take over considering quite a bit of the population is in some contact with cartels so it's kind of a guerilla war you'll have then where you'll deal with population that you don't know about and if they can be hostile or friendly. Mostly hostile ofcourse since you are invading their country.
Saw this in Iraq, Syria, Pakistan, Afghanistan. Random population just scattering about joining different sides, and you can't fire upon them unless you are fired upon so it will usually lead to a few casualties.
Oh well, feel like pure shit just want SOUTH AFRICA back