As soon as I'm elected president, Mexico WILL be assimilated. I don't think a war will be necessary. In fact, I daresay that a military presence will be overkill to the extreme. Their military will be all but jumping in joy at the prospect of becoming US military.
I just have to broker some deals with some crooks in charge over there and then I will own them all. The general population will follow along one way or another. How do you say "spin doctor" in spanish?
PS Mexico is rich in natural resources. Do the math.