The desire to uphold white rule has always been a potent force in American politics. And since the era of eugenics ushered in a fanatical obsession with breeding, that goal has been aided by rhetoric that inflames fears of the extinction of whiteness. From the anti-immigration statutes of the 1920s to anti-miscegenation laws to Trump’s Muslim ban, racists have sought to protect the boundaries of white racial purity. It’s a boundary often drawn in blood and bullets.

“The grim truth of the matter is this: The whole white race is exposed, immediately or ultimately, to the possibility of social sterilization and final replacement or absorption by the teeming colored races,” wrote the Ku Klux Klan officer and author Theodore Lothrop Stoddard, in his 1921 book The Rising Tide of Color Against White World-Supremacy. The white man, he wrote, “cannot withstand colored competition.” Ninety-eight years later, a similar dread of white “replacement” has motivated four mass shootings in the past year—a sentiment little changed over most of a century. But today the Internet has allowed the message to be transmitted globally and reduced the time it takes for violent events to inspire one another.

At the heart of white supremacy is a belief, first and foremost, that a coherent white “race” exists. Socially, whiteness has been flexible in the past: It has admitted Irish, Italians, and Poles, for example, over the course of a single century. Whiteness is a social invention defined primarily by whom it excludes. Against whiteness there stands the invention of blackness, which in US history has meant inferiority in a racial hierarchy. Slaves coming to the Americas were stripped of their native languages, names, cuisine, and very identities as citizens of distinct cultures. In the place of this history, America substituted a racial order in which blackness was its own identity, one designed to bear the brunt of white greed and prejudice. In order to maintain racial order, racist ideologues and their agents had to define the enemies of whiteness as weak enough to defeat but strong enough to present an existential threat. So within whiteness grew a terror of being outbred or forcibly interbred into diminution and eventual destruction. As Lothrop Stoddard wrote, “colored competition” and “Negro fecundity” present a threat to white supremacy—and the very existence of whiteness.

The invention of whiteness and blackness dates back to before America was a country. In 1661, Virginia passed a law that prohibited interracial marriage. In the Constitution, racial hierarchy was measured out in precise fractions: Each slave counted for three-fifths of a white person. But the racial paranoia of interbreeding was refined in the 18th and 19th centuries, as race scientists employed phrenology, craniometry, eugenics, and prejudice to justify an existing edifice of racism. Science drew dividing lines, and the Ku Klux Klan and other violent racists policed them. While both de jure and extrajudicial racism had material benefits—recent research has revealed the extent to which lynchings of black men enabled white men to steal land—protecting white women’s virtue served as a potent psychological excuse. But while terror of black sexuality was a useful proxy for economic domination and persecution by white supremacists, the last century has seen that fear manifest as a fear of white extinction. A manufactured panic of being outbred and rendered extinct drove the anti-immigration legislation of 1924 and the revival of the Ku Klux Klan in the 1920s.

The modern manifestation of the fear expressed by Lothrop Stoddard is a fear that immigration and interracial breeding constitute a “genocide” against the white race. Influenced by the global response to Nazism, the “genocide” terminology dates back to 1994, when the white supremacist David Lane wrote a short document called The White Genocide Manifesto. In it, he wrote down 14 words that neatly encapsulate both the urgency and fixation on breeding of white supremacist ideology. The slogan simply states, “We must secure the existence of our people and a future for white children.” Lane wrote the manifesto in prison, serving a sentence for driving a getaway car from the murder of a Jewish radio host. He’d been part of a white supremacist terrorist group called the Order. Since the manifesto’s publication, the “14 words” have become a slogan, a battle cry, and motivational motto in one. The slogan is so embedded in the white supremacist consciousness that just the number 14—often paired with 88, which stands for “Heil Hitler” (H is the eighth letter of the alphabet)—is frequently used as a thinly coded way for white supremacists to signal their affiliations.

In 2012, the French academic Renaud Camus published The Great Replacement, a book that draws on the racist 1973 French novel The Camp of the Saints, which depicts a dystopia in which Western civilization is overrun by fecund, nonwhite migrants. The Camp of the Saints is a racist horror story, with the thin scrim of fiction hanging over an ideology of white supremacy (it’s also one of Steve Bannon’s favorite books). The Great Replacement is its nonfiction counterpart, arguing the same thesis: that liberal immigration policy and anemic breeding by whites constitute a kind of suicide of the white West. In Camus’s worldview, this “genocide by substitution” is a deliberate betrayal by global elites, who wish to see a world “devoid of ethnic specificity.” His principal evidence was the existence of Muslims in France, where Muslims make up less than 10 percent of the population. Fear of a “great replacement” drove the infamous chants on August 12, 2017, in Charlottesville, Virginia: “You will not replace us,” the men shouted with their torches. And: “Jews will not replace us!”

The Great Replacement was also the title that Brenton Tarrant, the Australian man who shot dead 51 Muslims at prayer in Christchurch, New Zealand, gave his manifesto. Aptly, it began by repeating the same sentence three times: “IT’S ALL ABOUT THE BIRTHRATES.” Tarrant laid out a brief version of Camus’s thesis—that the white West, globally, was on the precipice of being destroyed by immigration and outbreeding—largely due to white women’s “sub-replacement fertility rate.” Only through “direct action”—mass murder—could his ideological comrades avert “white genocide.” He considered the 51 people he killed “invaders”—destroyers of New Zealand’s status as a “white country.” (Just over 1 percent of New Zealand’s population is Muslim.)

The same anti-immigrant paranoia—and the conviction that there exists a plot against whiteness, seeking to destroy it through immigration—has motivated three mass shootings in the United States. In October 2018, a young man named Robert Bowers murdered 11 Jews in a Pittsburgh synagogue; his social media posts revealed he was punishing Jews for, as he saw it, inducing the death of whiteness by orchestrating immigration. Six weeks later, John Earnest, 19, murdered a woman and wounded several more people at a synagogue in Poway, California, leaving a manifesto that blamed Jews for “funding politicians and organizations who use mass immigration to displace the European race.” And this past weekend, a white supremacist gunman murdered 20 people and wounded 26 more in an effort to stop what he called a “Hispanic invasion” of Texas. He was, he said, “defending my country from cultural and ethnic replacement.” All four mass shooters shared the same commitment to “great replacement” theory—a theory that rests on the racist commitment to white rule. While some adherents of “great replacement” theory are virulently anti-Semitic, attributing immigration to a Jewish plot, others point to a more generalized social decay as the cause of imminent white extinction. Increased acceptance of gay rights, feminism, and racial equality together form a plot—whether Jewish or not—to convince whites not to breed and allow nonwhites to overwhelm what had been white countries.

In America in 2019, “great replacement” theory has filtered from the margins to the mainstream. Donald Trump has made anti-immigrant rhetoric and policy the linchpin of his presidency. A fear of “invasion” was the centerpiece of the president’s electoral strategy during the 2018 midterms, centered around an immigrant “caravan” that was heading toward the Southern border. Harshly punitive policies toward asylum seekers and the detention of thousands in horrific conditions serve as propaganda for an administration that describes immigration as “infestation.” As the president put it in a 2018 tweet, his political opponents want “illegal immigrants, no matter how bad they may be, to pour in and infest our Country.” The Fox News–addled president has plenty of company on his favorite cable channel. As Splinter author Jack Mirkinson pointed out, Fox News’s rhetoric about immigration is a constant barrage of fears about “invasion”—or, as TV host Laura Ingraham put it in June, replacing the current American population, or swamping the current American population, with a new population of people.” It’s rhetoric that could have been drawn from a page of Camus, Lothrop Stoddard—or a white supremacist murder manifesto.

As in the 1920s, restrictive immigration policies are being passed in an environment of heightened societal racism. In the early 1920s, Congress passed the Asiatic Barred Zone Act, banning Asian immigration to the United States. Anti-miscegenation laws were passed in numerous states, and Virginia passed the Racial Integrity Act, which enforced a “one-drop” rule tightening the boundaries of whiteness. In this environment of politically acceptable racism, a revived Ku Klux Klan went on a racist terror campaign, committing murders, bombings, lynchings, and whippings. In 1921, a white mob attacked the Greenwood neighborhood of Tulsa, Oklahoma, a prosperous area known as “Black Wall Street.” The mob torched 35 city blocks, injured 800 people, and killed at least 36, though historians now estimate that as many as 300 people may have been murdered.

In the present day, the easy communication of a global web of radicalized young men and the instant dissemination of murderers’ manifestos have made mass shooting “socially contagious,” according to recent research. But if the desire for an infamous death is a social contagion, then so is the ideology that motivates white supremacist terrorism. Viewed through the warped lens of far-right message boards, news sites, chats, and YouTube channels, immigration is an imminent threat that demands a violent response. Young men marinating in the far-right Internet that glorifies murder, rape, and racism are driven to take up arms, to prevent “the doom of their race,” to be part of a larger cause and live on forever—if only on a list of murderers. Utilizing the terms “extinction” and “genocide,” “great replacement” theory imbues racism with existential urgency. When combined with a toxic online environment of unchecked incitement, the ideology acts as a catalyst for violence. Across the United States, communities are reeling from the results of an ideology whose rise is a call to slaughter.