WASHINGTON — In December 2014, a middle-aged man driving a car in Dijon, France, mowed down more than a dozen pedestrians within 30 minutes, occasionally shouting Islamic slogans from his window.

The chief prosecutor in Dijon described the attacks, which left 13 injured but no one dead, as the work of a mentally unbalanced man whose motivations were vague and “hardly coherent.”

A year and a half later, after Mohamed Lahouaiej Bouhlel slaughtered dozens of people when he drove a 19-ton refrigerated truck through a Bastille Day celebration on Thursday in Nice, France, the authorities did not hesitate to call it an act of Islamic terrorism. The attacker had a record of petty crime but no obvious ties to a terrorist group, yet the French prime minister swiftly said Mr. Lahouaiej Bouhlel was “a terrorist probably linked to radical Islam one way or another.”

Read more in The New York Times.