Mexico isn't Columbia, and 20K poorly equipped and trained "civilian" fighters are no match for well trained, well equipped US military personal. We've heard the same arguments for years about the terrain in Afghanistan and our (I'm from the US) guys seem to do quit well even on a level playing field (see the early days after 911 when the US dropped in small groups of CIA special forces).
I loved the West Wing but feel they relied too heavily on the "Vietnam/Jungle" angle when proposing the feasibility of an actual "drug war" in Columbia. Also, the West Wing was aired before many of the US's major military actions in the middle east, which have arguably proven that the US can combat an insurgency (if they care enough to that is) and can operate in an "unconventional" manor.
I think the worries of getting sucked into another Vietnam, or getting bogged down like the Soviets in Afghanistan have been debunked. As long as the political will is there, the US military can operate successfully in almost all combat scenarios.