It was 2004, and Sean McFate had a mission in Burundi: to keep the president alive and prevent the country from spiraling into genocide, without anyone knowing that the United States was involved. The United States was, of course, involved, but only through McFate's employer, the military contractor DynCorp International. Throughout the world, similar scenarios are playing out daily. The United States can no longer go to war without contractors. Yet we don't know much about the industry's structure, its operations, or where it's heading. Typically led by ex-military men, contractor firms are...