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The Château de Plieux, a fortified castle on a hilltop in the Gascony region of southwestern France, overlooks rolling fields speckled with copses and farmhouses. A tricolor flag snaps above the worn beige stone. The northwest tower, which was built in the fourteenth century, offers an ideal position from which to survey invading hordes. Inside the château’s cavernous second-story study, at a desk heavy with books, the seventy-one-year-old owner of the property, Renaud Camus, sits at an iMac and tweets dire warnings about Europe’s demographic doom.

On the sweltering June afternoon that I visited the castle, Camus—no relation to Albert—wore a tan summer suit and a tie. Several painted self-portraits hung in the study, multiplying his blue-eyed gaze. Camus has spent most of his career as a critic, novelist, diarist, and travel essayist. The only one of his hundred or so books to be translated into English, “Tricks” (1979), announces itself as “a sexual odyssey—man-to-man,” and includes a foreword by Roland Barthes. The book describes polyglot assignations from Milan to the Bronx. Allen Ginsberg said of it, “Camus’s world is completely that of a new urban homosexual; at ease in half a dozen countries.”

In recent years, though, Camus’s name has been associated less with erotica than with a single poignant phrase, le grand remplacement. In 2012, he made this the title of an alarmist book. Native “white” Europeans, he argues, are being reverse-colonized by black and brown immigrants, who are flooding the Continent in what amounts to an extinction-level event. “The great replacement is very simple,” he has said. “You have one people, and in the space of a generation you have a different people.” The specific identity of the replacement population, he suggests, is of less importance than the act of replacement itself. “Individuals, yes, can join a people, integrate with it, assimilate to it,” he writes in the book. “But peoples, civilizations, religions—and especially when these religions are themselves civilizations, types of society, almost States—cannot and cannot even want to . . . blend into other peoples, other civilizations.”

Camus believes that all Western countries are faced with varying degrees of “ethnic and civilizational substitution.” He points to the increasing prevalence of Spanish, and other foreign languages, in the United States as evidence of the same phenomenon. Although his arguments are scarcely available in translation, they have been picked up by right-wing and white-nationalist circles throughout the English-speaking world. In July, Lauren Southern, the Canadian alt-right Internet personality, posted, on YouTube, a video titled “The Great Replacement”; it has received more than a quarter of a million views. On great-replacement.com, a Web site maintained anonymously, the introductory text declares, “The same term can be applied to many other European peoples both in Europe and abroad . . . where the same policy of mass immigration of non-European people poses a demographic threat. Of all the different races of people on this planet, only the European races are facing the possibility of extinction in a relatively near future.” The site announces its mission as “spreading awareness” of Camus’s term, which, the site’s author concludes, is more palatable than a similar concept, “white genocide.” (A search for that phrase on YouTube yields more than fifty thousand videos.)

“I don’t have any genetic conception of races,” Camus told me. “I don’t use the word ‘superior.’ ” He insisted that he would feel equally sad if Japanese culture or “African culture” were to disappear because of immigration.

Camus’s partner arrived in the study with a silver platter, and offered fruitcake and coffee. Camus, meanwhile, told me about his “red-pill moment”—an alt-right term, derived from a scene in the film “The Matrix,” for the decision to become politically enlightened. As a child, he said, he was a “xenophile,” who was delighted to see foreign tourists flocking to the thermal baths near his home, in the Auvergne. In the late nineties, he began writing domestic travel books, commissioned by the French government. The work took him to the department of Hérault, whose capital is Montpellier. Although Camus was familiar with France’s heavily black and Arab inner suburbs, or banlieues, and their subsidized urban housing projects, known as cités, his experience in Hérault floored him. Travelling through medieval villages, he said, “you would go to a fountain, six or seven centuries old, and there were all these North African women with veils!” A demographic influx was clearly no longer confined to France’s inner suburbs and industrial regions; it was ubiquitous, and it was transforming the entire country. Camus’s problem was not, as it might be for many French citizens, that the religious symbolism of the veil clashed with some of the country’s most cherished secularist principles; it was that the veil wearers were permanent interlopers in Camus’s homeland. He became obsessed with the diminishing ethnic purity of Western Europe.

Camus supports the staunchly anti-immigrant politician Marine Le Pen. He denied, however, that he was a member of the “extreme right,” saying that he was simply one of many voters who “wanted France to stay French.” In Camus’s view, Emmanuel Macron, the centrist liberal who handily defeated Le Pen in a runoff, is synonymous with the “forces of remplacement.” Macron, he noted acidly, “went to Germany to compliment Mme. Merkel on the marvellous work she did by taking in one million migrants.” Camus derides Macron, a former banker, as a representative of “direct Davos-cracy”—someone who thinks of people as “interchangeable” units within a larger social whole. “This is a very low conception of what being human is,” he said. “People are not just things. They come with their history, their culture, their language, with their looks, with their preferences.” He sees immigration as one aspect of a nefarious global process that renders obsolete everything from cuisine to landscapes. “The very essence of modernity is the fact that everything—and really everything—can be replaced by something else, which is absolutely monstrous,” he said.

Camus takes William F. Buckley, Jr.,’s injunction to stand “athwart history, yelling Stop” to the furthest extent possible, and he can be recklessly unconcerned about backing up his claims. On a recent radio appearance, he took a beating from Hervé le Bras, a director emeritus at the Institut National d’Études Démographiques, who said that Camus’s proclamations about ethnic substitution were based on wildly inflated statistics about the number of foreigners entering France. Afterward, Camus breezily responded on Twitter: “Since when, in history, did a people need ‘science’ to decide whether or not it was invaded and occupied?”