Let's pull them out of these third world countries and let the do what they do well on our own border by protecting our own nation !
While that makes common sense, and would be economical,
I believe that to be a violation of " The Posse Comitatus Act of 1878 " , which forbids military troops from acting as police.
Technically, such a presence wouldn't be a police action, rather, it would be to end a civilian invasion at the southern border.