On social media, Richard Whitehead is a warrior for the American right. He has praised extremist groups. He has called for public executions of government officials he sees as disloyal to former President Donald Trump. In a post in 2020, he urged law enforcement officers to disobey COVID-19 public-health orders from “tyrannical governors,” adding: “We are on the brink of civil war.” Whitehead also has a day job. He trains police officers around the United States. Whitehead is part of a trend in pushing a radical-right political agenda to American police forces. He’s one of five police trainers identified by...