ISIS has claimed responsibility for the Paris attacks, the crash of the Russian airliner, and the bombings in Beirut. In his new critically acclaimed book The ISIS Apocalypse, Will McCants writes: “We’re used to thinking of al-Qaeda’s former leader Osama bin Laden as the baddest of the bad, but the Islamic State is worse.” What drives ISIS? What are its origins? Who are its leaders? How have they come so far? To answer these questions, we featured Will McCants, director of the Brookings Institution Project on US Relations with the Islamic World.