Meet Blackwater USA, the powerful private army that the U.S. government has quietly hired to operate in international war zones and on American soil. With its own military base, a fleet of twenty aircraft, and twenty-thousand troops at the ready, Blackwater is the elite Praetorian Guard for the "global war on terror"--yet most people have never heard of it.

Jeremy Scahill reports below on the Bush Administration's growing dependence on private security forces such as Blackwater USA and efforts in Congress to rein them in. This article is adapted from his new book, Blackwater: The Rise of the World's Most Powerful Mercenary Army (Nation Books).