It started in California, and it hasn't stopped yet.

On Saturday, a heavily armed 18-year-old white man rolled up to a supermarket in a predominantly Black neighborhood of Buffalo, N.Y. He had on military fatigues and body armor, and a camera strapped to his head in hopes of livestreaming his every move.

He parked his vehicle and then opened fire.

"This is the worst nightmare that any community can face," Buffalo Mayor Byron Brown said Saturday, "and we are hurting and we are seething right now."

Like the mayor, most of the victims were Black. Authorities say Payton Gendron of Conklin, N.Y., killed at least 10 people and wounded three others, some who were shopping, others who were working at the Tops Friendly Markets.

Maddeningly, such violence can happen anywhere in America these days — and usually does.

But it's the white supremacist explanation for it this time around, uncovered in a manifesto that authorities believe Gendron wrote and posted online, that should be all too familiar to Californians.

He is an adherent of the so-called Great Replacement theory. According to authorities, Gendron felt compelled to drive more than three hours to shoot innocent Black people indiscriminately with a high-powered rifle because white Americans are being "replaced" by people of color.

In many ways, this truly ugly conspiracy theory has some roots right here in the Golden State of the 1990s.

That's when some politicians, desperate to hold on to political power, were spreading fear and paranoia about millions of Mexican immigrants wanting resources and rights, and the inevitable decline of the state's white population.

Of course, the real origin of the "Great Replacement" theory is much older and inextricably linked to antisemitism, in that white supremacists blame Jews for nonwhite immigration. Hence, the chants of "Jews will not replace us" and "You will not replace us" by racists with tiki torches the night before the Unite the Right rally in Virginia in 2017.

The version of the theory making the rounds now posits not just that America is becoming more diverse, which is absolutely true, but that some secret cabal of elite Democrats is conspiring to bring in immigrants in any and every way possible to "replace" white Christian people and reshape American politics into some sort of secular, multicultural liberal image.

Never mind that Latino voters often sway conservative, as we saw in the 2020 presidential election, when Trump got a bigger share of that demographic's electorate than he did in 2016.

"Diversity is not a strength," Gendron wrote, according to snippets of the manifesto that authorities say he uploaded and are now floating around online. "Unity, purpose, trust, traditions, nationalism and racial nationalism is what provides strength."

We now know from that manifesto that Gendron traveled some 200 miles from his rural hometown to reach that supermarket in Buffalo because it was in a neighborhood with lots of Black people, authorities said.

Alongside racist, anti-immigrant rantings, the manifesto laid out how he planned to kill as many Black people as possible, authorities said. That he would shoot the security guard near the entrance before firing upon Black shoppers. That he had studied the floor plan and knew each aisle. What he would eat for lunch.

The FBI is investigating what happened as a "hate crime and racially motivated violent extremism." Erie County Sheriff John Garcia called the motive for the mass shooting "pure evil."

It's also a widespread, white supremacist ideology that has gone mainstream.

Late last year, a poll from the Associated Press-NORC Center for Public Affairs Research found that about a third of American adults believe an effort is afoot to "replace" U.S.-born Americans with immigrants.

In addition, roughly 3 in 10 think additional immigration will cause native, presumably white, Americans to lose their economic, political and cultural influence.

One reason is that irresponsible pundits keep touting the "Great Replacement" theory as an explanation for everything from the loss of manufacturing jobs in the Midwest to a spike in deaths from overdoses among white people addicted to painkillers.

It's a lie, and it's ridiculous and it's dangerous, especially in the era of social media. And yet, it never stops — even here.

As usual, California was the first — in this case, the first state forced — to confront these new population shifts. Political rhetoric and paranoia over a supposed "replacement" was the backlash to that diversity and the accompanying push for inclusiveness.

Many thought we had moved past it.

Now, decades later, as the rest of the country has started to look more like California, it's abundantly clear we haven't.

The "Great Replacement" theory also has been cited as a motive for numerous mass shootings in recent years, including at a Poway synagogue on the last day of Passover in 2019.

On Saturday, the Rev. Al Sharpton urged the White House to hold a meeting with Black, Jewish and Asian American leaders "to underscore the federal government (is) escalating its efforts against hate crimes."

President Joe Biden said the nation "must do everything in our power to end hate-fueled domestic terrorism" and called for a thorough investigation.

"A racially motivated hate crime is abhorrent to the very fabric of this nation," Biden insisted, against all logic, reason and recorded history.

Only time will tell if any of these statements will amount to more than the usual, empty "thoughts and prayers."

What's clear is we can't keep treating acts of white supremacy as one-off crimes committed by supposed lone wolves suffering from mental health problems. We also can't keep giving a pass to those who directly or indirectly encourage adherence to the "Great Replacement" theory or any other tenet of racism or extremism.

We've seen what happens when we allow such things to happen. We saw it in California in the 1990s and echoes of it remain here today.

A Buffalo City Council member on Saturday described the supermarket where so many were killed as along "a historical street for African Americans," in a tight-knit community a few miles outside of downtown.

I've set foot in many a Tops Friendly Markets in my life. The chain, which used to dot the street corners of my native Cleveland, was often filled with Black shoppers going about their lives, worried about replacing the milk in their refrigerators, not people.

"The depth of pain that families are feeling," Mayor Brown said, "and that all of us are feeling right now cannot even be explained."

Make it stop.

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Erika D. Smith is a columnist for the Los Angeles Times writing about the diversity of people and places across California.