It is hard to pick up a newspaper these days without reading about Army and Marine Corps recruiting and retention woes. Nonstop deployments and the danger faced by troops in Iraq are making it hard for both services to fill their ranks. The same goes for the National Guard and Reserves. (The Navy and Air Force, which are much less in harm’s way, have no such difficulty.)

Just to stay at their present sizes, the Army and Marines are shoveling money into more advertising, extra recruiters, and bigger enlistment bonuses. And yet it’s clear to everyone (except, that is, President Bush and Secretary of Defense Rumsfeld) that the U.S. military is far too small to handle all the missions thrown its way. We need to not only maintain the current ranks but also to expand them in order to recover from a 1990s downsizing in which the Army lost 300,000 soldiers.

Some experts are already starting to wonder whether the war on terrorism might break the all-volunteer military. But because reinstating the draft isn’t a serious option (the House defeated a symbolic draft bill last year, 402 to 2), some outside-the-box thinking is needed to fill up the ranks. In this regard, I note that there is a pretty big pool of manpower that’s not being tapped: everyone on the planet who is not an American citizen or permanent resident.

Since September 11, 2001, Bush has expedited the naturalization process for soldiers. But to enlist, the Pentagon requires either proof of citizenship or a green card. Out of an active-duty force of about 1.4 million, only 108,803 are foreign-born (7%), and 30,541 are noncitizens (2%).

This is an anomaly by historical standards: In the 19th century, when the foreign-born population of America was much higher, so was the percentage of foreigners serving in the military. During the Civil War, at least 20% of Union soldiers were immigrants, and many of them had just stepped off the boat before donning a blue uniform. There were even entire units, like the 15th Wisconsin Volunteer Infantry (the Scandinavian Regiment) and General Louis Blenker’s German Division, where English was hardly spoken.

The military would do well today to open its ranks not only to legal immigrants but also to illegal ones and, as important, to untold numbers of young men and women who are not here now but would like to come. No doubt many would be willing to serve for some set period in return for one of the world’s most precious commodities – American citizenship. Open up recruiting stations from Budapest to Bangkok, Cape Town to Cairo, Montreal to Mexico City. Some might deride those who sign up as mercenaries, but these troops would have significantly different motives than the usual soldier of fortune.

The simplest thing to do would be to sign up foreigners for the regular U.S. military, but it would also make sense to create a unit whose enlisted ranks would be composed entirely of non-Americans, led by U.S. officers and noncommissioned officers.

Call it the Freedom Legion. As its name implies, this unit would be modeled on the French Foreign Legion, except, again, American citizenship would be part of the “pay.” And rather than fighting for American security writ small – the way the Foreign Legion fights for the glory of France – it would have as its mission defending and advancing freedom across the world. It would be, in effect, a multinational force under U.S. command – but one that wouldn’t require the permission of France, Germany, or the United Nations to deploy.

The Freedom Legion would be the perfect unit to employ in places such as Darfur that are not critical security concerns but that cry out for more effective humanitarian intervention than any international organization could muster. American politicians, so wary (and rightly so) of casualties among American citizens, might take a more lenient attitude toward the employment of a force not made up of their constituents. An added benefit is that by recruiting foreigners, the U.S. military could address its most pressing strategic deficit in the war on terrorism – lack of knowledge about other cultures. The most efficient way to expand the government’s corps of Pashto or Arabic speakers isn’t to send native-born Americans to language schools; it’s to recruit native speakers of those languages.

Similar considerations early in the Cold War led Congress to pass the Lodge Act in 1950. This law allowed the Army Special Forces to recruit foreigners not living in America with the promise of citizenship after five years of service. More than 200 Eastern Europeans qualified as commandos before the Lodge Act expired in 1959. There’s no reason why we couldn’t recruit a fresh batch of foreigners today. It would certainly be easier than trying to sweet-talk more troops out of recalcitrant allies or, these days, recruiting at American high schools.

Mr. Boot is a senior fellow at the Council on Foreign Relations and a columnist for the Los Angeles Times, where this first appeared.