The Islamic State, known as ISIS, exploded into the public eye in 2014 with startling speed and shocking brutality. It has captured the imagination of the global jihadist movement, attracting recruits in unprecedented numbers and wreaking bloody destruction with a sadistic glee that has alienated even the hardcore terrorists of its parent organization, al Qaeda.

Jessica Stern and J.M. Berger, two of America's leading experts on terrorism, dissect the new model for violent extremism that ISIS has leveraged into an empire of death in Iraq and Syria, and an international network that is...