There are tens of thousands of them in Iraq. They work for companies with exotic and ominous-sounding names, like Crescent Security Group, Triple Canopy, and Blackwater Worldwide. They travel in convoys of multicolored pickups fortified with makeshift armor, belt-fed machine guns, frag grenades, and even shoulder-fired missiles. They protect everything from the U.S. ambassador and American generals to shipments of Frappuccino bound for Baghdad's Green...