A police forensic expert outside of a building where a 21-year-old Syrian refugee killed a woman with a machete and injured two other people in the city of Reutlingen, Germany, July 24, 2016.
Vincent Kessler / Reuters

A string of lone wolf terrorist attacks in France, Germany, the United States and elsewhere suggests that the phenomenon continues to spread and that it is growing increasingly lethal. Between October 2015 and August 2016 radicalized individuals, as well as “wolf packs,” carried out over 20 attacks in response to the Islamic State’s call to indiscriminately kill “nonbeliever” civilians.

The lone-wolf strategy benefits the Islamic State (also known as ISIS) in several ways. First, it is cheap and relatively easy. It requires no planning on its part or even contact with, or knowledge of, the perpetrators. Second, lone wolves frustrate preventive measures

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