What Drove a Mass Attack on Brazil’s Capital? Mass Delusion.

For the past 10 weeks, supporters of the ousted far-right President Jair Bolsonaro had camped outside Brazilian Army headquarters, demanding that the military overturn October’s presidential election. And for the past 10 weeks, the protesters faced little resistance from the government.

Then, on Sunday, many of the camp’s inhabitants left their tents in Brasília, the nation’s capital, drove a few miles away and, joining hundreds of other protesters, stormed Congress, the Supreme Court and the presidential offices.

By Monday morning, the authorities were sweeping through the encampment. They dismantled tents, tore down banners and detained 1,200 of the protesters, ferrying them away in buses for questioning.

Why an encampment demanding a military coup was allowed to expand for over 70 days was part of a larger set of questions that officials were grappling with on Monday, among them:

Why were protests allowed to get so close to Brazil’s halls of power? And why had security forces been so outnumbered, allowing throngs of protesters to easily surge into official government buildings?

Brazil’s justice minister, Flávio Dino, said various security agencies had met on Friday to plan for possible violence in the planned protests on Sunday. But, he said, the security strategy hatched in that meeting, including keeping protesters away from the main government buildings, was at least partly abandoned on Sunday and there were far fewer law enforcement officers than had been anticipated.

“The police contingent was not what had been agreed upon,” he said, adding that it was unclear why plans had changed.

Some in the federal government blamed the governor of Brasília, Ibaneis Rocha, and his deputies, suggesting that they had been either negligent or complicit in understaffing the security forces around the protests.

Late Sunday, Alexandre de Moraes, a Supreme Court justice, suspended Mr. Rocha from his job as governor for at least 90 days, saying that the upheaval “could only occur with the consent, and even effective participation, of the security and intelligence authorities.”

Whatever security lapses may have occurred, Sunday’s riot laid bare in shocking fashion the central challenge facing Brazil’s democracy. Unlike other attempts to topple governments across Latin America’s history, the attacks on Sunday were not ordered by a single strongman ruler or a military bent on seizing power, but rather were fueled by a more insidious, deeply rooted threat: mass delusion.

Millions of Brazilians appear to be convinced that October’s presidential election was rigged against Mr. Bolsonaro, despite audits and analyses by experts finding nothing of the sort. Those beliefs are in part the product of years of conspiracy theories, misleading statements and explicit falsehoods spread by Mr. Bolsonaro and his allies claiming Brazil’s fully electronic voting systems are rife with fraud.

Mr. Bolsonaro’s supporters have been repeating the claims for months, and then built on them with new conspiracy theories passed along in group chats on WhatsApp and Telegram, many focused on the idea that the electronic voting machines’ software was manipulated to steal the election. On Sunday, protesters stood on the roof of Congress with a banner that made a single demand: “We want the source code.”

Walking out of the protest encampment on Monday morning, Orlando Pinheiro Farias, 40, said he had entered the presidential offices on Sunday with fellow protesters to find documents related to “the investigations into the source code, which legitimize that Jair Messias Bolsonaro is the president of Brazil.”

He rattled off several government acronyms and secret investigations that he had read about on the internet, and then said that he had to go back to his tent to retrieve a Brazilian flag he had stolen from the building.

Delusions over the election extended to many protesters’ explanations of what had happened in the riots. People filing out of the encampment on Monday morning, carrying rolled-up air mattresses, extension cords and stools, each had a clear message: Mr. Bolsonaro’s supporters had not ransacked the buildings. Rather, they said, those causing the damage were radical leftists in disguise, bent on defaming their movement.

“Have you ever heard of the Trojan Horse?” said Nathanael S. Viera, 51, who had driven 900 miles to take part in the protests on Sunday. “The infiltrators went in and set everything up, and the damn press showed the Brazilian nation that we patriots are the hooligans.”

The scenes on Sunday of right-wing protesters draped in their national flag roaming through the halls of power were strikingly similar to those from the Jan. 6 storming of the United States Capitol, and so were the confused beliefs that drove protesters in both countries to invade federal buildings and film themselves doing so.

“Donald Trump was taken out with a rigged election, no question about it, and at the time he was taken out, I said, ‘President Bolsonaro is going to be taken down,’” said Wanderlei Silva, 59, a retired hotel worker standing outside the encampment on Monday.

Mr. Silva saw his own similarities between the riots on Sunday and those on Jan. 6, 2021. “The Democrats staged that and invaded the Capitol,” he said. “The same way they staged it here.”

Brazil has long seen itself in the mold of the United States: a sprawling, diverse country rich in natural resources, spread across a collection of independent states and governed by a strong central government. But its tumultuous political history never truly mimicked the American system, until the past several years.

“If there was no Trump, there would be no Bolsonaro in Brazil. And if there was no invasion of the Capitol, there wouldn’t have been the invasion we saw yesterday,” said Guga Chacra, a commentator for Brazil’s largest television network, who lives in New York and tracks politics in both countries. “Bolsonarismo tries to copy Trumpism, and Bolsonaro supporters in Brazil try to copy what Trump supporters do in the United States.”

Even a description of Brazil’s 2022 presidential election reads like a summary of the 2020 American one: a far-right populist incumbent with a penchant for insults and off-the-cuff tweets against a septuagenarian challenger on the left running on his proven political track record and a promise to unite a divided nation.

But the election’s aftermath was different.

While former President Donald J. Trump fought to overturn the results and urged his supporters to march on the U.S. Capitol on Jan. 6, Mr. Bolsonaro had effectively given up and decamped for Florida by the time his voters were forcing their way into the offices he once occupied.

Mr. Bolsonaro spent part of Monday in the hospital in Florida, dealing with abdominal pains stemming from a stabbing he suffered in 2018, his wife said on social media. Mr. Bolsonaro is planning to stay in Florida for the next several weeks or months, hoping investigations in Brazil into his activity as president will cool off, according to a friend.

Ned Price, the State Department spokesman, would not comment specifically on Mr. Bolsonaro’s visa status, citing privacy laws. But he said that any person who came to the United States under a diplomatic visa and who “is no longer engaged in official business on behalf of their government” was expected either to depart the country or request a different type of visa within 30 days.

“If an individual has no basis on which to be in the United States, an individual is subject to removal,” Mr. Price said.

In a recorded address in the final days of his presidency, Mr. Bolsonaro said that he had tried and failed to use the law to overturn the 2022 election, and suggested that his supporters should now move on. “We live in a democracy or we don’t,” he said. “No one wants an adventure.” On Sunday, he posted a message on Twitter, criticizing the violence.

But his years of rhetoric against Brazil’s democratic institutions — and his political strategy of instilling fear of the left in his supporters — had already left an indelible mark.

Interviews with protesters in recent weeks appeared to show that Mr. Bolsonaro’s movement was moving beyond him. It is now driven by deeply held beliefs among many right-wing Brazilians that political elites rigged the vote to install as president Luiz Inácio Lula da Silva, whom they regard as a communist who will turn Brazil into an authoritarian state like Venezuela.

Mr. Lula, the new president, is a leftist but is not a communist. And independent security experts said there was no evidence of irregularities in the 2022 vote. A separate analysis by Brazil’s military found just one potential vulnerability in Brazil’s fully digital voting system, which would require the coordination of multiple election officials to exploit, a scenario that security experts said was extremely unlikely.

Mr. Lula, who had campaigned on unifying the divided nation, is now faced with investigating and prosecuting many of his political opponents’ supporters just a week into his presidency. The authorities said that roughly 1,500 protesters had been detained by Monday evening, and that they would be held until at least the investigation was finished.

On Monday, Mr. Lula spoke with President Biden, who conveyed “the unwavering support of the United States for Brazil’s democracy and for the free will of the Brazilian people ,” White House officials said. Mr. Biden invited Mr. Lula to the White House in early February. (It took more than 18 months for him to meet with Mr. Bolsonaro at a summit in Los Angeles.)

In a televised speech on Monday night, Mr. Lula said that his government would prosecute anyone who had attacked Brazil’s democracy on Sunday. “What they want is a coup, and they won’t have one,” he said. “They have to learn that democracy is the most complicated thing we do.”

He and many of Brazil’s top government officials then walked together from the presidential offices to the Supreme Court, crossing the same plaza that a day before was thronged with mobs calling for the overthrow of his government.

Reporting was contributed by Ana Ionova, André Spigariol, Yan Boechat, Leonardo Coelho and Michael D. Shear.