It's a classic chicken-or-the-egg question. Has the presence of the National Guard contributed to historic lows of illegal immigrants trying to sneak into the US from Mexico? Or do the historic lows mean that billions spent on enforcement is a waste of money?

Today the Monitor looked at how the arrests of immigrants along the border are at their lowest level since 1972, according to US immigration officials. The numbers of those arrested in fiscal year 2011, which ended on Sept. 30, stood at 327,577, compared to 1.6 million back in 2000. This comes after the National Guard was deployed to the border, first under former President George W. Bush, and continued under President Obama.

The Washington Post recently looked at the results of the National Guard being sent to the border. As the paper calculates: “The 1,200 National Guard troops have helped Border Patrol agents apprehend 25,514 illegal immigrants at a cost of $160 million — or $6,271 for each person caught.”

Now, the Obama administration is preparing to cut down their presence, as reported by the Houston Chronicle. Relying less on manpower, the future mission will depend more on aerial surveillance and other measures. The details are to be announced later this month, but they are apparently precisely in response to fewer immigrants attempting to cross the border illegally.