The recent post "Other than Mexicans" briefly mentioned Fort Huachuca in southern Arizona. Yesterday the Washington Times published "Army base plagued by illegals' intrusions":

Fort Huachuca, a 150-square-mile U.S. Army base in Arizona just 20 miles north of the Mexican border, is home to the country's premier military intelligence school and, despite the sensitive nature of its assignment, has become a popular corridor for illegal migrants headed to America.

Law-enforcement authorities and congressional investigators said the illegals - some in groups of up to 60 - routinely wander through base housing units, drink from hoses and pools, and trample through the yards of military families and other private areas en route to nearby highways, where they catch northbound rides...

...In addition to having the nation's premier military intelligence school, Fort Huachuca hosts several other key military training commands and units from the Homeland Security Department.

You can probably imagine what would happen to you if you tried to traipse across a military base. (For entertainment purposes, here's an example from Area 51.) While Fort Huachuca is very large and it's located in rugged country, I'm sure more could be done if there were the will to do it. Apparently, not stopping the flow of "good-hearted", cheap, and exploitable labor is more important to Washington than things like securing a military base against incursions.