Last Oct. 7 was the deadliest day of violence against Jews since the end of the Holocaust. It was also the start of the most virulent eruption of antisemitism in the West since World War II. To mention but a single statistic, in the first three days following the ghastly Hamas pogrom, online calls for violence against Jews surged by 1,200 percent, according to the Antisemitism Cyber Security Monitoring System. The orgiastic spilling of Jewish blood drove antisemites into a frenzy of hatred that has yet to subside.

A year ago, it might have been possible to believe that the post-Holocaust taboo on openly loathing Jews was still in force. Though outbursts of antisemitic viciousness had been occurring in recent years with increasing and alarming frequency — and coming from both the right and the left — they seemed the exception to a broadly enforced rule. No longer. Even in America, antisemitism is being mainstreamed, particularly among the young. The post-Holocaust reprieve has ended. Antisemitism is back with a vengeance, brazen and self-assured.

Over the millennia, the hatred of Jews has assumed predictable patterns, none more fundamental than the “big lie.” Antisemites — whether they hate the Jewish people, the Jewish religion, or the Jewish state — have expressed their hatred not merely by exaggerating negative stereotypes, denying inconvenient truths, or recycling bigoted slurs. Time and again they have also promoted falsehoods so monstrous and vicious, so audaciously and aggressively untrue, that their utter implausibility gives them the illusion of truth.

“In the big lie there is always a certain force of credibility,” wrote Adolf Hitler in “Mein Kampf.” Many people “tell small lies in little matters,” he observed, but “it would never come into their heads to fabricate colossal untruths, and they would not believe that others could have the impudence to distort the truth so infamously.” That is the paradox and power of the big lie, especially when it is repeated often enough: Even reasonable minds gradually accept it as fact. After all, they subconsciously assume, why would government officials or the press or social elites make claims so shocking and scandalous unless they were true?

Hitler launched his war against the Jews with the big lie that Germany was defeated in World War I because it had been “stabbed in the back” by Jewish traitors. In reality, German Jews were exceptionally loyal to the fatherland — more than 100,000 had served in the German army during World War I and 12,000 had fallen in battle. Later came an even more unspeakable lie: the grotesque claim that Jews were racial subhumans who defiled the purity of Aryan blood. Today it seems incredible that citizens of Germany, the most sophisticated and cultured nation in Europe, could have believed such insane falsehoods. But tens of millions of Germans did believe them. And when the Nazis set about ostracizing and then exterminating Europe’s Jews, it was with the avid support and participation of innumerable German citizens.

Jewish history is replete with such deadly lies.

Beginning in the 12th century, Jews were adly accused of kidnapping and slaughtering Christian children, in order to use their blood for baking Passover matzah. On the strength of these “blood libels,” revived again and again in medieval Europe (and recycled to this day in the Muslim world), countless Jews were imprisoned, expelled, or murdered. The accusation was not just false, it was a wholesale inversion of the truth. In Jewish culture, the consumption of blood is forbidden; indeed, it is one of the first prohibitions in the Hebrew Bible. An essential step in preparing kosher meat is to meticulously rinse it clean of blood. Yet this demented big lie was believed by vast numbers of Europeans.

So was the big lie, spread during the devastating Black Death of the 14th century, that Jews had caused the plague by poisoning wells and springs. They intended, it was said, “to kill and destroy the whole of Christendom and have lordship over all the world.” The charge was ludicrous, not least because Jews were dying of the plague like everyone else. Yet so effective was the lie, and so fervent the antisemitic hunger to believe in Jewish culpability, that it resulted in thousands of Jews being rounded up and burned alive.

From era to era, the big lies told about Jews mutate. At the start of the 20th century, Jews were accused in the notorious “Protocols of the Elders of Zion” of conspiring to control the world. In the aftermath of 9/11, they were accused of having plotted the terrorist attacks. Louis Farrakhan has for years indoctrinated his Nation of Islam adherents in the libel that Jews were to blame for “the horror of the trans-Atlantic slave trade, plantation slavery, [and] Jim Crow.” A thriving industry denies that there was ever a Holocaust, insisting that the most comprehensively documented genocide in history is a Jewish fiction. All of these are lies so crazy, such obvious reversals of reality, that they ought to be impossible for any sane individual to believe.

Yet all of them have been believed, often in great numbers and by celebrated and influential people.

In the modern era, the most lethal antisemitic big lies have been those spread about Israel. The Jewish state has been slandered for decades as being founded on racism and practicing apartheid. Israel is smeared as a colonialist intruder with no legitimate right to exist. Long after it withdrew entirely from Gaza, it is routinely condemned as an “occupier.”

For decades, whenever Israel has been forced to fight in own defense, antisemites have raged that it slaughters civilians with a wanton disregard for decency and international law. In the year since Oct. 7, these lies have been taken to fantastic extremes — that Israel deliberately bombs hospitals, that it knowingly induces famine, and even, in an updated imitation of Holocaust denial, that it faked the Oct. 7 massacre for propaganda purposes and concocted the evidence of mass rape and sexual torture.

The biggest of these big lies is that Israel is committing genocide against the Palestinians. Once again, this is not just a falsehood but a wholesale inversion of reality. It is Hamas that unleashed genocidal violence against Israel on Oct. 7, Hamas whose leaders publicly vow to carry out the “slaughter, extermination, and annihilation” of the Jews, and Hamas whose strategy explicitly relies on maximizing Palestinian deaths. To believe that Israel — which has always gone to great lengths to avoid civilian casualties — is actually trying to exterminate the Palestinian people is about as logical as believing that Jews would spread the Black Death by poisoning the drinking water.

But when it comes to Jews, people — even educated and sophisticated people — have always been willing to believe the most monstrous big lies. And the horror of Oct. 7, far from making such lies seem more deranged than ever, has led even more people to embrace them.