An extremist group exploits the very democracy it opposes. Last Saturday, on the hottest Independence Day ever recorded in D.C., hundreds of masked men paraded around the U.S. Capitol while chanting “Reclaim America!” Members of the white-supremacist group Patriot Front arrived carrying Confederate and upside-down American flags. Pictures of the spectacle soon flooded social media, documenting their appearance, their march and their exit, including one arresting image captured inside a packed Metro subway car. In it, a sea of men in khaki hats, blue shirts and white neck gaiters pulled over their faces filled the aisle and every seat but...