Problem with this 'solution' is that an part of the US that mexico might acquire would be turned INTO a poverty ridden part of the third world within a decade.
Already happened.
See Los Angeles.
Of course. The third world isn't a place, it's a state of mind. Wherever there are third world people, there is the third world.
What they will do next, then, if we let them, is move north and eastward, like a plaque of locusts, devouring and destroying everything in their path.