ISIS, perhaps more than any other multinational terrorist group, has become known for its ability to attract Westerners away from their comfortable lives in prosperous countries and into the dangerous world of violent jihad. Peter Bergen, who was the first journalist to interview Osama bin Laden on television and has written five books on terrorism, explored what attracts Americans to radical Islam in his new book, "The United States of Jihad." There's no one-size-fits-all explanation for why Westerners join foreign terrorist organization, nor is there one common profile for an American jihadist. "Each case is a little bit different, and...