Andrew Neil of BBC kept asking Labour Party leader Jeremy Corbyn the same question -- over and over.

"Eighty percent of Jews think that you're anti-Semitic," he stressed. "Wouldn't you like to take this opportunity tonight to apologize to the British Jewish community for what's happened?"

Corbyn would not yield: "What I'll say is this -- I am determined that our society will be safe for people of all faiths."

The Daily Express called this late-2019 clash a "horror show." This BBC interview, with surging fears of public anti-Semitism, lingered in headlines as Brits went to the polls. Corbyn's party suffered its worst defeat in nearly a century.

Meanwhile, in America, a wave of anti-Semitic attacks left Jews wondering if it was safe to wear yarmulkes and symbols of their faith while walking the sidewalks of New York City. In suburban Monsey, a machete-waving attacker stabbed five people at a Hasidic rabbi's Hanukkah party. Finally, thousands of New Yorkers marched to show solidarity with the Jewish community.

The NYPD estimates that anti-Semitic crimes rose 26% last year. Anti-Semitic hate crimes in New York, Los Angeles and Chicago are expected to hit an 18-year high, according to research at California State University, San Bernardino. 

No one who watches the news can doubt that "the darkness has returned. It has returned likewise to virtually every country in Europe," argued Rabbi Lord Jonathan Sacks, who led the United Hebrew Congregations of the Commonwealth from 1991-2013. He was knighted by Queen Elizabeth in 2005 and entered the House of Lords.

"That this should have happened within living memory of the Holocaust, after the most systematic attempt ever made … to find a cure for the virus of the world's longest hate -- more than half a century of Holocaust education and anti-racist legislation -- is almost unbelievable. It is particularly traumatic that this has happened in the United States, the country where Jews felt more at home than anywhere else in the Diaspora."

Why now? In an essay for the Jewish Telegraphic Agency, Rabbi Sacks urged religious and political leaders to study trends -- often digital -- behind these tragedies.

"Anti-Semitism, or any hate," he argued, "becomes dangerous in any society when three things happen: when it moves from the fringes of politics to a mainstream party and its leadership; when the party sees that its popularity with the general public is not harmed thereby; and when those who stand up and protest are vilified and abused for doing so."

Imagine the hellish "Protocols of the Elders of Zion" updated for the Internet. In the age of smartphones and viral videos, noted Sachs, millions of people can brew hate online -- rarely speaking face-to-face with their disciples or their victims. This gap creates what researchers call a "disinhibition effect" that turns up the heat.

"Cyberspace has proved to be the most effective incubator of resentment, rancor and conspiracy theories ever invented," noted Sacks. Most people "encounter these phenomena … in the privacy of their own home. This allows them to be radicalized without anyone realizing it is happening. Time and again, we read of people carrying out horrific attacks, while those who knew them recall not having seen any warning signs that they were intent on committing evil attacks."

It's crucial to grasp the logic behind political and cultural fears on both the left and the right. Many people are furious because they believe the "world as it is now is not the way it used to be, or ought to be," he argued.

"The far left has not recovered from the global collapse of communism and socialism as ideologies. … The far right feels threatened by the changing composition of Western societies, because of immigration on an unprecedented scale and low birth rates among the native population. … Many radical Islamists are troubled by dysfunctions in the Muslim world."

Thus, many people around the world want to know why bad things are happening. Anyone seeking to fight anti-Semitism, Sacks wrote, needs to understand what can go wrong with that process.

"When bad things happen, good people ask, 'What did I do wrong?' … Bad people ask, 'Who did this to me?' They cast themselves as victims and search for scapegoats to blame. The scapegoat of choice has long been the Jews."