In the most recent wave of assaults, over the past six months, attackers have targeted Jews as they worshiped at Pittsburgh’s Tree of Life synagogue in October and at the Chabad of Poway near San Diego during the end of Passover; Christians at Easter services in Sri Lanka; and Muslims praying in Christchurch, New Zealand.

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The nature and frequency of these attacks have raised urgent questions about how to fight extremism in a time of political polarization, largely unregulated social media and diminished trust in community organizations, including religious and political institutions.

“The main culprits are social media and the total collapse of any sense of bipartisanship about how to fight hate in America,” said Rabbi Abraham Cooper, director of global social action at the Simon Wiesenthal Center, a Jewish human rights organization. “Evildoers around the world learned from the 9/11 terrorists that you don’t need the backing of a state or a mass movement. And then individuals who are probably psychiatric cases are inspired to do these things because social media spreads a culture of hate in the most public ways, and encrypted communications allows them to go private to discuss the how-tos.”

The suspect in the Poway shooting, a 19-year-old nursing student who was arrested and identified by police as John T. Earnest, apparently published a manifesto online, an increasingly common way for assailants to spread their ideologies and offer their versions of an explanation for the unthinkable. This one is a lengthy, rambling document markedly similar to the one posted by the avowed white supremacist who was charged with killing 50 people in New Zealand last month.

In the manifesto published online Saturday under the name John Earnest, the writer said he attacked the synagogue to “help the European race” defend itself against “international Jewry.” Much of the statement is a litany of conspiracy theories about Jews that have been at the heart of anti-Semitism for the past two millennia: killing Jesus, controlling finance and the media, “promoting race mixing.”

In Poway, a suburb of 50,000 people northeast of San Diego that bills itself as “the city in the country,” the shooting seemed to many on Sunday like a violent injection of the political poison they had so far experienced mainly through the news media.

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“It feels so close,” said Kadie Nagy, as she and her 3-year-old son, Sky, nestled a small bouquet in honor of the shooting victims near a light post piled high with flowers and cards. Sobbing, she said she didn’t know the victims and is not Jewish, but she felt a connection to those who worshiped at Chabad, an Orthodox denomination that is part of the highly observant Hasidic movement.

“Each of these different violent acts, they just feel like they’re getting closer and closer to us,” she said. “We love this city. We love our Jewish friends.”

Poway’s population is 81 percent white, according to U.S. Census data, with 16 percent of the city reporting Hispanic heritage. Thirteen percent of residents are Asian and 2 percent black.

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As with many anti-Semitic terrorists through history, the manifesto claimed to eschew political ideology and lashed out in several directions. It called President Trump “anti-white,” bashed conservatives and liberals alike, and dismissed Hispanics, Muslims and blacks with the usual slurs. “There is no love without hatred,” it asserted.

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The writer quoted extensively from the New Testament — a common tactic among the new crop of terrorists, who often use religious justifications for their violence.

“They attack symbols that are sacred and held dear,” said Jonathan Greenblatt, chief executive of the Anti-Defamation League (ADL). “It’s a way of attacking a broader community.”

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The shootings and bombings at places where people go to feel unified, comforted and inspired are, in one sense, nothing new. Religion, one of history’s most powerful uniting forces, always has served as a source of contention and bloodshed. From the Crusades to the Holocaust, nations, leaders and movements have waged war against people because of what they do or don’t believe.

But the nature of the violence inflicted on people because of their faith has changed sharply in recent years, swerving from nations deploying armies to individuals and small groups mounting seemingly random attacks against noncombatants.

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That pivot has taken place at a time when popular attitudes toward people of other religions have actually softened in much of the world, according to public opinion surveys.

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In the United States, for example, there’s been a sharp decline in anti-Semitic attitudes during the past half-century, according to ADL studies — a finding echoed in a number of European surveys.

“Anti-Semitic attitudes have been historically low, but it’s difficult now to feel anything other than alarm,” Greenblatt said. “Society is just more complicated — technology has empowered individuals and has disempowered states.”

Greenblatt said the surge in anti-Semitic crimes — up 37 percent in 2017, according to the FBI, a sharper rise than crimes against any other group — is a leading indicator of hate crimes against many others as well. “Although this typically starts with the Jews, it never ends with the Jews,” he said. “It’s like a disease that engulfs the broader population.”

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Religious and political leaders called on Trump to make a clear statement against the white-supremacist rhetoric that has surrounded recent attacks.

“The president needs to put out the tweet that this kind of behavior and this kind of language is outside the acceptable,” Cooper said.

Trump opened a political rally in Wisconsin on Saturday night with a strong statement of exactly that kind. “We forcefully condemn the evil of anti-Semitism and hate,” he said in Green Bay. “It must be defeated. We will all get to the bottom of it.”

Cooper noted that although the Islamic State, a chief ideological enemy of the United States, was defeated on the battlefield, “they were untouched, unchallenged in their online marketing.”

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“What we’re seeing are the old hatreds, dusted off and supercharged through social media,” Cooper said. “It’ll cost them a few pennies, but we need the big boys — Twitter, Facebook, Google, ­Instagram, YouTube — to make sure this weapon they’ve created isn’t inspiring this phenomenon to multiply.”

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Jewish institutions across the country assured their members Sunday that they are beefing up security. “Patrols and police presence have been increased at synagogues and other Jewish institutions in our area,” said a statement from the Jewish Federation of Greater Washington.

The group, like many, put the Poway attack in the context of the string of assaults on religious facilities: “Houses of worship of all faiths . . . reflect the highest ideals and aspirations of our democracy and of the human spirit,” the federation said. “We must put a stop to the encroaching normalization of the kind of attacks suffered today at Chabad, at the Tree of Life synagogue, and at mosques, churches, Sikh temples, and other worship spaces.”

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In the nation’s Jewish communities, especially in places where Jews are a tiny minority, the Poway shootings generated a new wave of fear.

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Anna Gerratana came to the United States from Venezuela in 2015, leaving a town where the small number of Jews felt they “had to be hiding all the time,” she said. Gerratana, her husband and their young daughter settled in Cary, N.C., where they attend a Chabad synagogue.

Here, she thought, they would be safe. But in October, anti-Semitic fliers festooned with swastikas began circulating in their neighborhood. Police charged the son of a judge with “ethnic intimidation.”

“It’s like there’s no place in the world where you can be really comfortable with the idea of being a Jew,” said Gerratana, who is 32.

Chabad synagogues generally operate on a principle of radical openness to Jews of all denominations and to non-Jews. The shootings had some members wondering how to maintain that openness given the threat of violence.

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“We have an open-door policy; there’s always strangers showing up,” said Dobie Thaler, who runs a Chabad house on the University of Wisconsin at Milwaukee campus with her husband, a rabbi. “But are we supposed to start questioning everyone that walks in?”

“Students or friends ask me, because they’re afraid, ‘Should I wear my star of David necklace?’ ” Thaler said. She told them: “If you hide it, then the hate is winning.”

From earliest history, religions preached the idea of a single human community, a universal order. But from the start, people have faced off against one another according to the nature of their worship. Nations put religious symbols on the flags their armies carried into battle and claimed new lands in the name of their faith and their deity.

Even as the centuries went by and new ideas of tolerance and ecumenism promised a time of brotherhood, “all the experience of history confirms that sharing the same beliefs has been a preliminary to quarreling about their interpretation,” as historian Theodore Zeldin wrote in “An Intimate History of Humanity.”

Now, in a time when national governments in many parts of the world struggle to retain authority and trust and the Internet allows individuals to construct their own communities of interest and affinity, the ability of lone extremists to wreak havoc is greater than ever before.

“If you’re a young person and you want to change history, it can seem so simple now,” Cooper said. “If you have the notion that you need to save white Christian America, then the targets are going to be the Muslims and the Jews. If we’re going to stop that, we’re going to have to change the nature of social media, and we need clarion voices to insist on the fundamentals of truth and justice.”

In Poway, the hard path to solutions seemed distant, the shock of the violence too fresh.

“It’s just amazing to see someone that young filled with that much hate, wanting to go out there and do something like that,” said Linnéa Sellers, 21, who was working at a gas station near the Chabad synagogue. “It sounded like he was happy to do so, which absolutely baffled me.”