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

Instead, more than from 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 worldview expressed in a document that authorities say they 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 civilization 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!’ ”