The Islamic State is not a threat to the homeland "at this point," Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told USA TODAY. Unchecked, however, it will threaten Israel and Europe, he said, describing the group's world view as virtually "apocalyptic. …

The extent and sophistication of the Islamic State's use of social media, video and other communication tools is unprecedented in terrorism, possibly because the group has so many tech-savvy Westerners in its ranks.

Its message is half Disney, half Game of Thrones. "They say, 'Greetings from the Caliphate! Come to the Caliphate!' '' Bloom says. "They make it sound so inviting.'' That appeals to some recruits; the images posted online of massacres, executions and severed heads apparently appeal to others.

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