Mexico is the number 3 oil supplier to the U.S.
LINK
The U.S. is Mexico's top trading partner.
http://www.dallasfed.org/research/sw.../swe0601c.html
Mexico supplies cheap labor to the U.S. (personal opinion, no link).
I suspect effective control of the border will not occur until the above conditions change. Therein lies the possibility of trouble.