Influencing distant attackers to pledge allegiance to the Islamic State and then carry out mass murder has become a core part of the group’s propaganda over the past two years. It is a purposeful blurring of the line between operations that are planned and carried out by the terror group’s core fighters and those carried out by its sympathizers. This public oath is about the only requirement that the Islamic State imposes on followers who wish to carry out acts of terror in its name. In an annual speech, the terror group’s spokesman, Abu Muhammad al-Adnani, last month incited its...