Benjamin Ryan Teeter was at his home in Hampstead, N.C., when the call to action came. It was an alert from the heart of the raging protests in Minneapolis, posted on an online forum by a fellow member of the Boogaloo movement, a loosely knit group of heavily armed, anti-government extremists. The "alert" was from a man who had a run-in with the Minneapolis police while on the frontline of the police-brutality protests set off by the death of George Floyd. "He caught mace to the face," said Teeter, and "put out a national notice to our network." After Teeter...