In our piece written last year for Foreign Policy following the New Year’s Day attack in New Orleans, we called attention to the highly lethal model of the Islamic State in recent years, wherein supporters use the group’s brand, tactics, and online guides to conduct attacks, often using simple, low-tech methods to unleash carnage—a franchising model leading to “inspired” rather than “directed” attacks. The past year of successful and thwarted external operations confirms the threat of a globally dispersed group with strong ideological resonance beyond its territorial hot spots. Shortly after the New Orleans attack, a 23-year-old Syrian asylum-seeker killed...