The world has been preoccupied with the size of the so-called Islamic State terror organization since it sprang up in 2014. But, for all of that preoccupation, there has been precious little certainty about just how big ISIL is, who’s actually on the ground fighting and how it all impacts U.S. national security. In part two of our exclusive interview, National Counterterrorism Director Nicholas Rasmussen breaks down the composition of ISIL, including participation by Americans.