I'm a little confused here..
Last time I checked, we had a Canadian border as well..
If for whatever reason, Obama suffered a brainfart and agreed to military enforcement, at least on our side of the Border with Mexico, what makes you think Drug pushers/human traffickers/Gun Smugglers won't just try a different Venue {if they're not already doing it}, Saskatchewan, Manitoba & British Columbia seem to have big enough borders to try stuff, no?
Then of course, there's the Pacific Coast, East Coast, Gulf of Mexico coast...
If you think they won't bother figuring how to smuggle things by Water, you need to spend some time on the East coast and west coast of Puerto Rico, Jamaica & Dominican Republic/Haiti...
So, what you're suggesting is, Military presence at all these Borders?
I think I've seen that somewhere...
http://en.wikipedia.org/wiki/Korean_Demilitarized_Zone