Last week Roseanne Barr — who, with the hit reboot of her show, has become one of the most prominent Donald Trump supporters in the country — tweeted that the president has freed hundreds of children a month from sexual bondage. “He has broken up trafficking rings in high places everywhere,” she wrote. (The tweet has since been deleted.) Barr’s tweet, puzzling to the casual observer, was a reference to QAnon, an expansive, complicated pro-Trump conspiracy theory. The theory is fascinating as an artifact of our current political derangement, but more than that, it’s profoundly revealing about the lengths to...