I'm not sure it's even for that --- they've been on the border well before 9-11 ---- Mexicans don't think they're there for good reasons. Many say they're protecting "their own" drug cartel from the competition or helping them get their deliveries made. The Mexican army is also used to keep the people in control ---- and while smuggling drugs into the USA is seen as a good thing for Mexico, smuggling weapons into Mexico is seen as a bad thing for the oligarchs they're paid to protect.
Wow, that's interesting. Mexicans don't think they're there for good reasons.
I agree......probably protecting both drug & people smugglers