Many commentators have labelled the racist ideology apparently behind the horrific attack in Buffalo on Saturday a conspiracy theory. It is easy to believe that such extremism is a product of our internet age, where conspiracies run rampant, hatred blooms in the dark corners of the web and young men can be radicalised without even leaving their bedrooms. If only the problem were so simple.

Instead, more than a conspiracy theory, the attack stemmed from the White supremacy that is deeply rooted in the White American political tradition. In fact, core features of the world view expressed in a document that authorities believe was written by the gunman — fear of an immigrant invasion changing the character of the country, an obsession with White birthrates, imagining Black Americans as a demographic cancer within the body politic — often have been the stuff of mainstream White politics in the United States.

The internet age may be the occasion for the resurgence of these ideas, but it is not the cause. The roots of the violence are ideological, not technological. And only confronting the racism and genocidal fantasies that have animated White American politics for generations will address the problem.

These fantasies began in a triumphalist mode of conquest. In 1845, John O’Sullivan spoke for many in the burgeoning White republic when he described “an irresistible army of Anglo-Saxon emigration” and proclaimed its “manifest destiny to overspread the continent”.

As the supposed science of race matured in the ensuing decades, experts spoke of race as the key that unlocked the very meaning of history. Historian William Swinton’s popular textbook declared, “History proper concerns itself with but one highly developed type of mankind . . . the Caucasians form the only truly historical race. Hence we may say that civilisation is the product of the brain of this race”.

White people made history; everyone else was a bystander to it.

When the influential minister Josiah Strong published Our Country in 1885, fantasies of White conquest had gone global. Strong gleefully described God’s chosen Anglo-Saxon race descending on the aborigines of the global south and eradicating everyone in its way. “It would seem as if these inferior tribes were only precursors of a superior race, voices in the wilderness crying: ‘Prepare ye the way of the Lord!’”

As Strong’s blending of racial and religious themes suggest, violent White conquest was usually imagined as both a racial and divine right. If White civilisation was entitled to ownership of the Earth, it was because God had willed it to be so. One pro-Ku Klux Klan minister succinctly declared this faith in White America’s dual religio-racial tradition when he declared, “our ancestors . . . came to this continent and fell first on their knees, then upon the aborigines . . .”

But by the mid-1880s, there was a note of disquiet and fear alongside these proclamations of racial triumph by divine will. For the age of mass migration had begun, and millions of people from Eastern and Southern Europe — who were seen as ethnically and religiously inferior to Anglo-Saxon Protestants — were entering the United States. Strong’s book was a plea to Americanise and evangelise these immigrants as quickly as possible, lest the nation come to ruin.

In the early 20th century, these fears intensified. In 1905, Japan shocked the White world when it won a war against Russia. Meanwhile, goods, ideas and people were on the move at unprecedented scale. Fearing a demographic “onslaught”, across the world White countries acted to fortify the borders of racial hierarchy in the next decades‚ either by seeking to exclude or expel people deemed racially inferior.

Political leaders believed ensuring White demographic dominance was essential because racial theorists of the time linked racial identities to political capacities. Anglo-Saxons were by nature rational political beings with a capacity for self-government. As Madison Grant, a pioneering conservationist and one of the leading race scientists of the era intoned, the “Nordics are, all over the world, a race of soldiers, sailors, adventurers and explorers, but above all, of rulers, organisers and aristocrats ...” Other races were by nature fit for more passive roles in the history of human affairs.

It is easy to dismiss Grant as a quaint example of prejudice. But he gave voice to widespread concerns. American political elites worried that foreign peoples did not appreciate or understand the nation’s institutions. Indeed, they wondered if such peoples were even capable of self-government.

That made the possibility of White demographic eclipse scary. For all its dominance, the White population was outnumbered globally. The Harvard-trained historian Lothrop Stoddard sounded the alarm in The Rising Tide of Color: The Threat Against White World Supremacy. Such men were hardly at the extremist fringe. Stoddard had his work published in leading mass circulation magazines such as the Saturday Evening Post and Collier’s.

While fortifying borders against the threat abroad, leading American racial theorists fretted about the challenges at home — namely racially inferior African-Americans threatening to destroy the republic from within and low White birthrates. The pioneering feminist Charlotte Perkins Gilman expressed common Progressive-era elite sentiment when she described Black Americans not as integral members of American life but as “a large body of aliens” who represented a problem for American civilisation. She proposed a system of state-directed forced labour for all Black Americans who did not meet “a certain grade of citizenship”. Many other experts went even farther: those deemed inferior should not be allowed to reproduce. More than two dozen states enacted sterilisation programmes targeting African-Americans and other populations imagined as inferior.

But that would not address low White birthrates. Decadence had left White men refined but effeminate, and White women educated but fragile. Black and immigrant women were having more children than White women, threatening the future of the republic. Teddy Roosevelt warned that “the competition between the races” was a “warfare of the cradle” and “no race has any chance to win a great place unless it consists of good breeders as well as of good fighters”. During his presidency, Roosevelt popularised the idea of “race suicide” and chided Americans for selfishly limiting the size of their families.

Popular lore claims the Nazis discredited this racist political culture. After the Holocaust, few could now doubt the dangers of racist ideology. But in reality, this racist thinking continued to animate White politics in the decades after the Second World War.

In 1975, French author Jean Raspail published an English-language edition of The Camp of the Saints, a genocidal fantasy about Europe destroying an immigrant invasion from the global south. The book has remained an inspiration in White supremacist circles ever since. But the book’s popularity was not limited to the fringe. National Review, the flagship magazine of the postwar American conservative movement, reviewed the book favourably. Dartmouth professor Jeffrey Hart wrote that the book invited the reader to realise that “killing a million or so starving refugees from India would be a supreme act of individual sanity and cultural health”.

In recent decades, especially after the civil rights movement changed America’s political culture in important ways, popular media narratives often assumed a vast distance between a racist past and a more enlightened present. Such narratives of progress highlighted real gains, but also made it harder to see troubling continuities in White American political culture. Simplistic tales of White racism as a relic of the past help explain why the misleading “lone wolf” framework continues to drive media reporting on White terrorism. These narratives keep us from asking the questions we ask after other sorts of terrorist attacks. In reality, White attackers are embedded in thick ideological contexts of sympathy and support in their communities.

Today’s White supremacist terrorism is the sick afterlife of yesterday’s White supremacist conquest. This terrorist violence is what happens after triumphalist narratives of White global conquest have curdled into fear, paranoia and defensiveness. Far from a fringe conspiracy, this legacy affects all Americans and implicates many of us. Defeating the politics of hate can happen only with an honest assessment of our past — one that stops lionising past champions of such politics. Without doing so, these theories will continue to breed violence and tragedy.

• Jesse Curtis is assistant professor of history at Valparaiso University and author of The Myth of Colorblind Christians: Evangelicals and White Supremacy in the Civil Rights Era