The summer of 2020 resembled a political revolution. Militants associated with Black Lives Matter and Antifa led the most destructive riots in American history. Radicals burned down city blocks, perpetrated acts of violence, and briefly established an “autonomous zone” in Seattle characterized by a police-free political order and a homicide rate 50 times higher than that of Chicago. For those who monitor left-wing radical movements, this shouldn’t have been a complete surprise. Federal law enforcement had long acknowledged the threat of left-wing, anarchist, and racialist violence. In 2016, the FBI published a report raising the alarm about the threat of...