Three years ago today, a 21-year-old man drove nearly 700 miles from his hometown of Allen, Tex., to a Walmart in El Paso. The store, just a few miles north of the U.S.-Mexico border, was a common destination for Mexican shoppers who drove across the international boundary to buy cheap goods—some locals called it the “Mexican Walmart.” Its clientele and proximity to Mexico is what made the man choose that particular Walmart. In a manifesto published on 8chan earlier that day, he wrote that he wanted to “shoot as many Mexicans as possible.” Using an AK-47-style rifle he ordered from Romania, he killed 23 people and injured dozens more.

In the hours following the El Paso shooting, journalists attempted to piece together a motive. The manifesto, titled “An Inconvenient Truth,” made it easy enough. In it, the alleged shooter claimed the attack was a “response to the Hispanic invasion of Texas.” Analyses of the manifesto highlighted its similarities to then-President Trump’s rhetoric. A year earlier, Trump had described a caravan of Central American asylum seekers headed to the United States as an “invasion.”

In his manifesto, the gunman said his views on immigration predated Trump’s presidency. They were also more explicit. Where Trump has said Democrats want migrants to “infest” the United States and suggested George Soros funded the 2018 migrant caravan, he shooter wrote of the so-called “great replacement,” the white supremacist belief that migrants are being imported to Europe, the United States, and Australia to “replace” white people. Immigrants, he wrote, were dangerous because of their sheer numbers—they were a threat to not only the white race but the planet as well.

“The environment is getting worse by the year,” he wrote. “Most of y’all are too stubborn to change your lifestyle. So the next logical step is to decrease the number of people in America using resources. If we can get rid of enough people, then our way of life can be sustainable.” Though he acknowledged that white Americans consume more resources than immigrants and people of color, the shooter balked at the idea of killing his own people. By targeting a predominately Mexican-American community, he reasoned, he was simultaneously solving the problems of overpopulation and rampant immigration. Citing the mass shooter who attacked two mosques in Christchurch, New Zealand, as an inspiration, the El Paso shooter declared himself an eco-fascist.

The shooting in El Paso wasn’t an isolated incident, and experts who study white supremacist movements say eco-fascism is becoming “a more accepted part of the ideology.” The man who killed 10 people at a grocery store in Buffalo, N.Y., was also a self-proclaimed eco-fascist. Like the El Paso shooter, he went out of his way to target a community of color; like the El Paso shooter, he published a manifesto in which he claimed that demographic change, overpopulation, and climate change were linked. (Notably, some sections of that document were directly lifted from the Christchurch shooter’s own manifesto.) As historian Kathleen Belew explained shortly after the El Paso shooting, these attacks aren’t isolated incidents, and the people who perpetrate them aren’t lone wolves, no matter how many neighbors describe them as bullied outcasts. Most importantly, the shootings aren’t the end goal—they’re intended as wake-up calls, invitations for others to do the same.

In the years following the Christchurch and El Paso shootings, mainstream conservatives have quickly accepted the main tenets of “great replacement” theory. A May poll found that nearly 70 percent of Republican voters believe that US demographic change “has been motivated by progressive and liberal leaders actively trying to leverage political power by replacing more conservative white voters.” The idea that immigrants are to blame not only for demographic change but also for rising global temperatures has yet to trickle down to the conservative mainstream—Republicans are still coming around to the idea that climate change exists at all—but it’s already spreading beyond the movement’s violent fringe.

Tucker Carlson, one of the most popular talking heads in the country, has promoted the idea that immigration hurts the environment for more than five years. In an August 2018 segment, he claimed that one of the reasons he’s “so against illegal immigration” is because he “hate[s] litter.” A few months later, he said immigrants make the country “poor and dirtier.” That cost him advertisers, but garnered support from conservatives eager to accuse liberal immigration advocates of hypocrisy. “The left doesn’t want to talk about environmental problems that are tied directly to a specific policy that a majority of Americans want changed,” Quentin Borges-Silva, an Environmental Protection Agency communications officer, wrote for The Federalist. (Another piece by Borges-Silva, published in The Daily Caller, warned Americans that they had to choose between “mass immigration or endangered species.”)

This neo-Malthusian line of reasoning isn’t novel. I wrote about the long-standing historical links between nativism and environmentalism before the shootings in Christchurch and El Paso. In his book Border Walls Gone Green, John Hultgren notes that the environmental and immigration restriction movements emerged around the same time—in the late 19th century—and involved many of the same key players. Environmentalists grew increasingly concerned with overpopulation in the 1960s and ’70s, and fears of overpopulation soon gave way to a desire to limit both legal and illegal immigration.

In 1979, John Tanton, former president of the group Zero Population Growth, left the organization to start one of his own: the Federation for American Immigration Reform (FAIR), which seeks to reduce both illegal and legal immigration in order to “manage growth, address environmental concerns, and maintain a high quality of life.” (Sidney Swensrud, the former CEO of Gulf Oil, was a cofounder.) Two splinter groups, the Center for Immigration Studies (CIS) and NumbersUSA, came later. Together, the three groups helped tank immigration reform plans under Presidents Bush and Obama, and influenced Trump’s immigration policies. Though the Tanton network organizations have mostly chosen to frame immigration as an issue of assimilation—a pared-down version of “great replacement” rhetoric—they still publish the occasional press release about the harms migrants pose to the environment. After The Washington Post noted similarities between the El Paso shooter’s manifesto and FAIR and CIS’s own publications, both groups distanced themselves from the shooting. (CIS executive Mark Krikorian, however, told the Post that the manifesto was “remarkably well-written for a 21-year-old loner.”)

Given the conservationist and immigration restrictionist movements’ shared histories, it’s foolish to think of the desire to protect the environment as inherently left-wing. For Carlson and his ilk, connecting immigration and environmental degradation is a perfect way to own the libs. Mainstream Republicans’ climate denial is irrelevant to Carlson’s brand of nativist pseudo-environmentalism, as is the fact that, year after year, Democrats have voted to increase funding for ICE and Customs and Border Protection.

John Hultgren’s prediction that nativists would try to win over leftists by framing immigration as an environmental issue hasn’t quite occurred. Instead, some on the far right have accepted that climate change poses a threat to human existence—and they’ve decided that the best way to stop it is to not only restrict immigration but also to cull non-white populations by any means necessary. Democrats aren’t prepared for this shift. They’re used to conservatives’ denying the existence of climate change. But soon enough, Republicans may concede that climate change is real, and that as a result, we need to keep the border closed.

Trump’s approach to Covid-19 was a preview of how this strategy could work. He went from downplaying the pandemic’s severity to using it as a pretext to shut down the asylum system overnight. More than two years later, the Biden administration has kept pandemic restrictions in place at the border. Every time Biden has hinted at lifting them, a chorus of Republicans (and a few Democrats) have urged him to keep it in place.

We can no longer afford to write off eco-fascism as a far-right fringe ideology. Instead, we should think of the El Paso and Buffalo mass shooters as the militant wing of an increasingly organized movement. The alleged El Paso shooter’s trial is scheduled for June 2023; if found guilty, he faces the death penalty, as could the alleged Buffalo shooter. While both men may be convicted of criminal charges, the larger movement will be undaunted, and needs to be challenged on an ideological level. You can’t prosecute your way out of white supremacy, and focusing only on the most egregious attackers lets their sympathizers off the hook. In the same way that anti-abortion activists’ more socially acceptable lobbying and politicking is supplemented by an armed militant movement that has killed abortion doctors and bombed reproductive health clinics, anti-immigrant think-tanks like FAIR and CIS are providing ideological cover for mass murder in the name of environmentalism.