Robert Bowers wanted everyone to know why he did it.

“I can’t sit by and watch my people get slaughtered,” he posted on the social-media network Gab shortly before allegedly entering the Tree of Life synagogue in Pittsburgh on October 27 and gunning down 11 worshippers. He “wanted all Jews to die,” he declared while he was being treated for his wounds. Invoking the specter of white Americans facing “genocide,” he singled out HIAS, a Jewish American refugee-support group, and accused it of bringing “invaders in that kill our people.” Then–Attorney General Jeff Sessions, announcing that Bowers would face federal charges, was unequivocal in his condemnation: “These alleged crimes are incomprehensibly evil and utterly repugnant to the values of this nation.”

The pogrom in Pittsburgh, occurring just days before the 80th anniversary of Kristallnacht, seemed fundamentally un-American to many. Sessions’s denunciation spoke to the reality that most Jews have found a welcome home in the United States. His message also echoed what has become an insistent refrain in the Donald Trump era. Americans want to believe that the surge in white-supremacist violence and recruitment—the march in Charlottesville, Virginia, where neo-Nazis chanted “Jews will not replace us”; the hate crimes whose perpetrators invoke the president’s name as a battle cry—has no roots in U.S. soil, that it is racist zealotry with a foreign pedigree and marginal allure.

Warnings from conservative pundits on Fox News about the existential threat facing a country overrun by immigrants meet with a similar response. “Massive demographic changes,” Laura Ingraham has proclaimed, mean that “the America we know and love doesn’t exist anymore” in much of the country: Surely this kind of rhetoric reflects mere ignorance. Or it’s just a symptom of partisan anxiety about what those changes may portend for Republicans’ electoral prospects. As for the views and utterances of someone like Congressman Steve King (“We can’t restore our civilization with somebody else’s babies”), such sentiments are treated as outlandish extremism, best ignored as much as possible.

The concept of “white genocide”—extinction under an onslaught of genetically or culturally inferior nonwhite interlopers—may indeed seem like a fringe conspiracy theory with an alien lineage, the province of neo-Nazis and their fellow travelers. In popular memory, it’s a vestige of a racist ideology that the Greatest Generation did its best to scour from the Earth. History, though, tells a different story. King’s recent question, posed in a New York Times interview, may be appalling: “White nationalist, white supremacist, Western civilization—how did that language become offensive?” But it is apt. “That language” has an American past in need of excavation. Without such an effort, we may fail to appreciate the tenacity of the dogma it expresses, and the difficulty of eradicating it. The president’s rhetoric about “shithole countries” and “invasion” by immigrants invites dismissal as crude talk, but behind it lie ideas whose power should not be underestimated.

The seed of Nazism’s ultimate objective—the preservation of a pure white race, uncontaminated by foreign blood—was in fact sown with striking success in the United States. What is judged extremist today was once the consensus of a powerful cadre of the American elite, well-connected men who eagerly seized on a false doctrine of “race suicide” during the immigration scare of the early 20th century. They included wealthy patricians, intellectuals, lawmakers, even several presidents. Perhaps the most important among them was a blue blood with a very impressive mustache, Madison Grant. He was the author of a 1916 book called The Passing of the Great Race, which spread the doctrine of race purity all over the globe.

Grant’s purportedly scientific argument that the exalted “Nordic” race that had founded America was in peril, and all of modern society’s accomplishments along with it, helped catalyze nativist legislators in Congress to pass comprehensive restrictionist immigration policies in the early 1920s. His book went on to become Adolf Hitler’s “bible,” as the führer wrote to tell him. Grant’s doctrine has since been rejuvenated and rebranded by his ideological descendants as “white genocide” (the term genocide hadn’t yet been coined in Grant’s day). In an introduction to the 2013 edition of another of Grant’s works, the white nationalist Richard Spencer warns that “one possible outcome of the ongoing demographic transformation is a thoroughly mis-sa-gen-nated, and thus homogeneous and ‘assimilated,’ nation, which would have little resemblance to the White America that came before it.” This language is vintage Grant.

Most Americans, however, quickly forgot who Grant was—but not because the country had grappled with his vision’s dangerous appeal and implications. Reflexive recoil was more like it: When Nazism reflected back that vision in grotesque form, wartime denial set in. Jonathan Peter Spiro, a historian and the author of Defending the Master Race: Conservation, Eugenics, and the Legacy of Madison Grant (2009), described the backlash to me this way: “Even though the Germans had been directly influenced by Madison Grant and the American eugenics movement, when we fought Germany, because Germany was racist, racism became unacceptable in America. Our enemy was racist; therefore we adopted antiracism as our creed.” Ever since, a strange kind of historical amnesia has obscured the American lineage of this white-nationalist ideology.

Jews troubled Grant the most. “The man of the old stock,” he later wrote in The Passing of the Great Race, is being “driven off the streets of New York City by the swarms of Polish Jews.” But as the title of his 1916 work indicated, Grant’s fear of dispossession ran wide and deep:

These immigrants adopt the language of the native American, they wear his clothes, they steal his name, and they are beginning to take his women, but they seldom adopt his religion or understand his ideals and while he is being elbowed out of his own home the American looks calmly abroad and urges on others the suicidal ethics which are exterminating his own race.

Grant was not the first proponent of “race science.” In 1853, across the Atlantic, Joseph Arthur de Gobineau, a French count, first identified the “Aryan” race as “great, noble, and fruitful in the works of man on this earth.” Half a century later, as the eugenics movement gathered force in the U.S., “experts” began dividing white people into distinct races. In 1899, William Z. Ripley, an economist, concluded that Europeans consisted of “three races”: the brave, beautiful, blond “Teutons”; the stocky “Alpines”; and the swarthy “Mediterraneans.” Another leading academic contributor to race science in turn-of-the-century America was a statistician named Francis Walker, who argued in The Atlantic that the new immigrants lacked the pioneer spirit of their predecessors; they were made up of “beaten men from beaten races,” whose offspring were crowding out the fine “native” stock of white people. In 1901 the sociologist Edward A. Ross, who similarly described the new immigrants as “masses of fecund but beaten humanity from the hovels of far Lombardy and Galicia,” coined the term race suicide.

But it was Grant who synthesized these separate strands of thought into one pseudo-scholarly work that changed the course of the nation’s history. In a nod to wartime politics, he referred to Ripley’s “Teutons” as “Nordics,” thereby denying America’s hated World War I rivals exclusive claim to descent from the world’s master race. He singled out Jews as a source of anxiety disproportionate to their numbers, subscribing to a belief that has proved durable. The historian Nell Irvin Painter sums up the race chauvinists’ view in The History of White People (2010): “Jews manipulate the ignorant working masses—whether Alpine, Under-Man, or colored.” In The Passing of the Great Race, the eugenic focus on winnowing out unfit individuals made way for a more sweeping crusade to defend against contagion by inferior races. By Grant’s logic, infection meant obliteration:

The cross between a white man and an Indian is an Indian; the cross between a white man and a Negro is a Negro; the cross between a white man and a Hindu is a Hindu; and the cross between any of the three European races and a Jew is a Jew.

What Grant’s work lacked in scientific rigor, it made up for in canny packaging. He blended Nordic boosterism with fearmongering, and supplied a scholarly veneer for notions many white citizens already wanted to believe. Americans’ gauzy idealism blinded them, he argued, to the reality that newcomers from the Mediterranean and eastern Europe—to say nothing of anyone from Asia or Africa—could never hope to possess the genetic potential innate in the nation’s original Nordic inhabitants, which was the source of the nation’s greatness. Grant gleefully challenged foundational ideas:

We Americans must realize that the altruistic ideals which have controlled our social development during the past century and the maudlin sentimentalism that has made America “an asylum for the oppressed,” are sweeping the nation toward a racial abyss. If the Melting Pot is allowed to boil without control and we continue to follow our national motto and deliberately blind ourselves to all “distinctions of race, creed or color,” the type of native American of Colonial descent will become as extinct as the Athenian of the age of Pericles, and the Viking of the days of Rollo.

His thesis found eager converts among the American elite, thanks in no small part to his extensive social connections. The New York Times and The Nation were among the many media outlets that echoed Grant’s reasoning. Teddy Roosevelt, by then out of office, told Grant in 1916 that his book showed “fine fearlessness in assailing the popular and mischievous sentimentalities and attractive and corroding falsehoods which few men dare assail.” In a major speech in Alabama in 1921, President Warren Harding publicly praised one of Grant’s disciples, Lothrop Stoddard, whose book The Rising Tide of Color Against White World-Supremacy offered similar warnings about the destruction of white society by invading dusky hordes. There is “a fundamental, eternal, inescapable difference” between the races, Harding told his audience. “Racial amalgamation there cannot be.”

Harding’s vice president and successor, Calvin Coolidge, found Grant’s thesis equally compelling. “There are racial considerations too grave to be brushed aside for any sentimental reasons. Biological laws tell us that certain divergent people will not mix or blend,” Coolidge wrote in a 1921 article in Good Housekeeping.

Perhaps the most important of Grant’s elite admirers were to be found among members of Congress. Reconstruction struggles; U.S. expansion in the Philippines, Puerto Rico, and Hawaii; high levels of immigration—each had raised the specter of white people losing political power and influence to nonwhite people, or to the wrong kind of white people. On Capitol Hill debate raged, yet Republicans and Democrats were converging on the idea that America was a white man’s country, and must stay that way. The influx of foreigners diluted the nation with inferiors unfit for self-government, many politicians in both parties energetically concurred. The Supreme Court chimed in with decisions in a series of cases, beginning in 1901, that assigned the status of “nationals” rather than “citizens” to colonial newcomers.

America has always grappled with, in the words of the immigration historian John Higham, two “rival principles of national unity.” According to one, the U.S. is the champion of the poor and the dispossessed, a nation that draws its strength from its pluralism. According to the other, America’s greatness is the result of its white and Christian origins, the erosion of which spells doom for the national experiment.

People of both political persuasions like to tell a too-simple story about the course of this battle: World War II showed Americans the evil of racism, which was vanquished in the 1960s. The Civil Rights Act and the Voting Rights Act brought nonwhites into the American polity for good. The Immigration and Nationality Act of 1965 forever banished the racial definition of American identity embodied in the 1924 immigration bill, forged by Johnson and Reed in their crusade to save Nordic Americans from “race suicide.”

The truth is that the rivalry never ended, and Grantism, despite its swift wartime eclipse, did not become extinct. The Nazis, initially puzzled by U.S. hostility, underestimated the American commitment to democracy. As the Columbia historian Ira Katznelson writes in Fear Itself: The New Deal and the Origins of Our Time (2013), the South remained hawkish toward Nazi Germany because white supremacists in the U.S. didn’t want to live under a fascist government. What they wanted was a herrenvolk democracy, in which white people were free and full citizens but nonwhites were not.

That nations make decisions about appropriate levels of immigration is not inherently evil or fascist. Nor does the return of Grantian ideas to mainstream political discourse signal an inevitable march to Holocaust-level crimes against humanity. But to recognize the homegrown historical antecedents of today’s rhetoric is to call attention to certain disturbing assumptions that have come to define the current immigration debate in America—in particular, that intrinsic human worth is rooted in national origin, and that a certain ethnic group has a legitimate claim to permanent political hegemony in the United States. The most benignly intentioned mainstream-media coverage of demographic change in the U.S. has a tendency to portray as justified the fear and anger of white Americans who believe their political power is threatened by immigration—as though the political views of today’s newcomers were determined by genetic inheritance rather than persuasion.

The danger of Grantism, and its implications for both America and the world, is very real. External forces have rarely been the gravest threat to the social order and political foundations of the United States. Rather, the source of greatest danger has been those who would choose white purity over a diverse democracy. When Americans abandon their commitment to pluralism, the world notices, and catastrophe follows.