“This attack is a response to the Hispanic invasion of Texas.” Those were the words that appeared in a manifesto published shortly before the deadly shooting in El Paso on Saturday. More than half of so-called “alt-right killers” are motivated by the “white replacement” theory, which refers to the belief that white people will be systematically replaced by black and brown migrants. The killer in El Paso, who law enforcement believes authored the memo, is apparently no exception.

The white replacement theory is actually made up of two sub-conspiracies: “the great replacement” theory, which originated in France, and “the white genocide theory”, which comes from the US. Together, the theories are among the most widespread ideologies in far-right spaces, and the primary catalysts of far-right mass violence.

The great replacement can generally be understood as two core beliefs. The first is that “western” identity is under siege by massive waves of immigration from non-European/non-white countries, resulting in a replacement of white European individuals via demographics. The second is that replacement has been orchestrated by a shadowy group as part of their grand plan to rule the world – which they will do by creating a completely racially homogenous society. This group is often overtly identified as being Jews, but sometimes the antisemitism is more implicit.

These beliefs have proliferated in mass killer texts for the past eight years. They are generally understood as having begun with Anders Breivik, the Norwegian mass shooter whose 1,500-page manifesto expressed a fear of white ethnic replacement by migrants from the Middle East and North Africa. This same fear cropped up in the manifestos of several more mass killers in Europe, before making its way to the rambling screed published by the Christchurch shooter, which was titled The Great Replacement. The Christchurch manifesto begins with: “It’s the birthrates. It’s the birthrates. It’s the birthrates.” It was directly inspired by Breivik – and he, in turn, inspired the El Paso shooter.

Rene Aguilar and Jackie Flores pray at a makeshift memorial for the victims of Saturday’s mass shooting at a shopping complex in El Paso, Texas, on Sunday. Photograph: Andres Leighton/AP

The El Paso shooter begins his text by writing: “In general, I support the Christchurch shooter and his manifesto. This attack is a response to the Hispanic invasion of Texas. They are the instigators, not me. I am simply defending my country from cultural and ethnic replacement brought on by an invasion.”

Despite is recent proliferation, the great replacement theory was first popularized decades ago in the 1973 novel Le Camp des Saints (The Camp of the Saints) by Jean Raspail – a vastly influential book in contemporary white supremacist discourse.

In this work of speculative fiction, Raspail paints an apocalyptic picture of the complete collapse of all western society and culture stemming from a “tidal wave” of immigration from the “third world”. Over the course of the 20th century, the theory proliferated in different white supremacist and ethno-exclusion spaces. It was in 2010, however, that the great replacement theory truly took flight.

The white supremacist Renard Camus introduced the term in his book De l’Innocence, warning of the replacement of white Europeans by peoples coming from the Middle East and North Africa. This is the text that influences much of the white supremacist discourse that we see today, and fuels the growing “identitarian” movement around the world. Identitarians advocate for an ethnically and racially heterogeneous world; they believe that racial mixing (ie sex and reproduction between people of different races) weakens the fabric of our society and is an imminent threat to the stability of majority-white, western nations – as well as the world.

The great replacement is spreading like a virus; we must find a way to inoculate against it

This idea is echoed by the El Paso shooter, who writes that he is “against race mixing because it destroys genetic diversity and creates identity problems … Cultural diversity diminishes as stronger and/or more appealing cultures overtake weaker and/or desirable ones.” You may have heard about identitarians protesting against Muslim immigration in Berlin, or forcibly blocking asylum seekers from disembarking from boats in the Mediterranean. Identitarians point to the great replacement as both a direct threat and a key motivator.

Though the idea began in Europe, it has certainly found fertile ground for xenophobia and racism in the United States. Popularized by far-right social media personalities who populate the darker corners of YouTube, Reddit, Gab and even Twitter (I won’t be naming or linking to them, so as to prevent amplification – you can read this report instead), the great replacement theory has taken root in the US.

The US has its own identitarian movement now, the hate group Identity Evropa, and the great replacement theory has become immensely popular among a breadth of rightwing hate groups. The phrase “Jews will not replace us!”, chanted by neo-Nazis at Charlottesville, was in direct reference to the belief that white replacement is being orchestrated by a shadowy Jewish elite.

One of the reasons that the great replacement theory was able to take hold so firmly in the US was because of the history of white replacement conspiracies here. The US has its own theory, called the “white genocide” conspiracy, which came about in the Reconstruction-era after the abolition of slavery and constitutes a belief that the US is on the brink of a “race war”, in which freed slaves would rise up and kill their former masters. This belief has cropped up again and again throughout the 20th century (perhaps you will remember it from the Manson family murders), and most recently has been expressed in the manifestos of white supremacist killers like Dylann Roof and Frazier Glenn Miller.

The gap between the two theories, however, is closing. As white replacement theory propagates online (galvanized by anti-immigrant rhetoric from far-right populists the world over, from Trump to Hungary’s Viktor Orbán), so does the belief in an all-encompassing “white, European identity” in need of saving. This is not a purely US-based conspiracy, but rather a call to arms to protect what is seen as the white race on a transnational level.

An interfaith vigil for victims of a mass shooting which left at least 20 people dead, on Sunday in El Paso, Texas. Photograph: Mario Tama/Getty Images

In the memo believed to have been written by the El Paso shooter, he wrote: “I can no longer bear the shame of inaction knowing that our founding fathers have endowed me with the rights needed to save our country from the brink of destruction. Our European comrades don’t have the gun rights needed to repel the millions of invaders that plaque [sic] their country. They have no choice but to sit by and watch their countries burn.”

Though it is difficult to write about without giving platform to these mass shooters and their ideas, it is important to understand precisely what beliefs are galvanizing many of the mass shootings we are seeing today. It is important to understand that white replacement is a transnational idea and discourse, influencing killers from Germany, to New Zealand, to here in the US. It is a widespread fear of ethnic replacement, shifting to suit the context of the place in which is presents. In the US, that is a fear of ethnic replacement by migrants from South and Central America. It is also made much more deadly by the US’s epidemic of available guns – which has led to 251 mass shootings in 2019 alone.

The great replacement is a deadly conspiracy – as well as one that is immensely popular on social media and among fearmongers like Tucker Carlson, whether it is overtly referred to or merely dog-whistled. It is vital that we understand the origins and implications of the theory, even as we strive to diminish its platform. The great replacement is spreading like a virus; we must find a way to inoculate against it.