When I first made this suggestion, I got a lot of positive responses but also some scathing critiques. A retired Army sergeant in Houston wrote (expletives deleted): "Are you out of your mind? The last thing we need in our military is a bunch of illegal immigrants serving in combat operations for a country to which they are not culturally bonded!" But there is no better way to build that bond than through military training and discipline. Drill sergeants have been forging cohesive units out of disparate elements since the days of the Roman legions.
In the past, the U.S. military had many more foreigners than we do today. (During the Civil War, at least 20% were immigrants. Now it's 7%.) The British army, among many others, has also made good use of noncitizens. Nepalese Gurkhas still fight and die for the Union Jack despite not being "culturally bonded" to it. No doubt they would do the same for the Stars and Stripes.
Some letter writers invoke the specter of mercenaries leading to the fall of the U.S. as they supposedly led to the fall of Rome. That's a misreading of Roman history. As classicist Victor Davis Hanson points out, by the 1st century AD, the legions "were mostly non-Italian and mercenary, and the empire still endured for nearly another 500 years." If only the Pax Americana were to last half as long!
Other critics think it's repugnant to ask foreigners to face dangers that citizens won't. But there is always an element of unfairness in war. Unless you institute a truly universal draft (we've never done it), some will always be more at risk than others.
Besides, the U.S. already makes ample use of mercenaries. We rely on tens of thousands of contractors in Iraq, Colombia and elsewhere, many of them not Americans. They would be a lot more useful if they were in uniform and subject to military orders so that we could avoid mix-ups like the one that just happened in Iraq, where Marines detained 19 employees of an American engineering firm for allegedly firing on them.
Would foreigners sign up to fight for Uncle Sam? I don't see why not, because so many people are desperate to move here. Serving a few years in the military would seem a small price to pay, and it would establish beyond a doubt that they are the kind of motivated, hardworking immigrants we want.
Anyway, what's the alternative? $100,000 signing bonuses? Recruiting felons?