A woman places flowers at a memorial for the victims of a suicide attack at a concert by Ariana Grande that killed more than 20 people Monday night in central Manchester, Britain, Wednesday, May 24, 2017. (AP/Emilio Morenatti)

Why Stopping Worldwide Terrorism Is So Challenging

The terrorist threat in the United States is episodic, sporadic and inconsistent. Too often policies react to fear rather than real threat estimates.