This morning, the Islamic State’s semi-official news agency, Amaq, took credit for the Las Vegas massacre, which killed 58 and wounded another 515. The likely killer, identified by police as 64-year-old Stephen Paddock of Mesquite, Nevada, was not known to be a supporter of the Islamic State, or indeed a Muslim of any type. For now, the only evidence that the Islamic State was involved is its own assurance—first a press release announcing that a “soldier of the Islamic State” executed the concertgoers, and a follow-up for the baffled, explaining that he converted to Islam months ago. The FBI has...