The CIA claims it has as many as 31,000 fighters in Syria and Iraq, and Massoud Barzani, president of Iraq's Kurdistan Regional Government, thinks the number may be as high as 200,000.

When ISIS fighters conquered the Iraqi city of Mosul last year, they stole enough materiel to supply three fighting divisions, including up-armored American Humvees, T-55 tanks, mobile Chinese artillery pieces, Soviet anti-aircraft guns, and American-made Stinger missile systems. ISIS controls a swath of territory the size of Great Britain and is expanding into Libya and Yemen.

ISIS: Inside the Army of Terror, by Michael Weiss and Hassan Hassan, paints a gripping and disturbing picture of this new "caliphate" in the Levant and Mesopotamia.