In his new book The ISIS Apocalypse: The History, Strategy, and Doomsday Vision of the Islamic State (St. Martin's Press, 2015), William McCants explains how the Islamic State has been so successful at attracting followers and conquering land.

McCants is a fellow at the Center for Middle East Policy at the Brookings Institution and director of the Project on U.S. Relations with the Islamic World, as well as an adjunct faculty member at Johns Hopkins University.

What we need is not more boots on the ground, but more sandals (locals) says @will_mccants on fighting ISIS. pic.twitter.com/ISBzCcfqGj

— Brian Lehrer Show (@BrianLehrer) September 24, 2015