In the weeks after Trump’s election, an up-and-comer on the far right named Richard Spencer appeared on NBC, NPR, and CNN. Profiles of him appeared in the Washington Post and the Los Angeles Times. More were in the works. Showtime was putting out feelers for a documentary. Then, just before Thanksgiving, Spencer gave a speech at the annual conference of his think tank, the National Policy Institute. NPI was dedicated to the “preservation of the heritage, identity, and future of European people in the US and elsewhere.” Spencer might have ended his speech declaring that America belonged to Europeans. But...