At around 10 o’clock on an April night in 1941, a strange radio program began broadcasting from Germany. American listeners tuning in to a shortwave station heard the melody of “Yankee Doodle” and the clopping of horse hooves, followed by a man speaking. “Tonight I, an American observer, come galloping on the radio,” the first broadcast began.

Every night but Saturdays after that, a man calling himself Paul Revere would address the nation in a vitriolic rant of pro-Nazi—and anti-British and anti-Roosevelt—propaganda.

About a month after the show launched, on the night of May 26, 1941, the host announced that it was his 52nd birthday. He was now, he said, “the exact age of Adolf Hitler, the most successful man in the world.” Then he introduced himself as Douglas Chandler, a contributor to National Geographic magazine.

At National Geographic headquarters in Washington, D.C., a memo was circulating among top editors with a newspaper clipping from The Washington Post: “Nazi ‘Paul Revere’ Proves Former Baltimore Man.” Letters began pouring in from magazine subscribers who had been listening to the disparaging broadcasts, in which Chandler had boasted about his employer while repeatedly railing against its leadership.

“I couldn’t have been more surprised if it had been the Annual Report of the Ladies’ Benevolent Society being ripped to ribbons!” wrote one concerned reader, who had heard the magazine mentioned a dozen times. “This is what I am curious about—how did the Nazis come to have such a spite against the ‘National Geographic’?”

Both National Geographic and the FBI had already been investigating Chandler’s Nazi ties. But now, with his radio reveal, the American public discovered that a National Geographic writer had—loudly—taken up the Nazis' cause. ...