Jeffrey Herf is distinguished university professor of history at the University of Maryland. His books include "The Jewish Enemy: Nazi Propaganda During World War II and the Holocaust."

On the evening after the deadliest act of violence against American Jews, President Trump said that Jews had “endured terrible persecution” for centuries: “You know that. We have all read it. We have studied it.”

But his response to the horrific news that a gunman had killed 11 people and wounded six at Shabbat services at Tree of Life synagogue in Pittsburgh made it clear that Trump doesn’t actually understand the nature of anti-Semitism at all.

After the shooting, on the airport tarmac on his way to speak to the National FFA Organization, Trump didn’t utter a single word about anti-Semitism. Instead, he resorted to the talking points of the National Rifle Association regarding the need for armed guards in the synagogue, as if an armed guard with a pistol could have been a match for this lunatic armed, yet again, with an AR-15-style assault rifle. (It’s worth noting here that three armed police officers were injured.)

A few hours later, Trump did state the obvious: “It looks definitely like it’s an anti-Semitic crime.” Then he said something that a modestly well-informed president would not say: “That is something you wouldn’t believe could still be going on.”

You “wouldn’t believe” it only if you were clueless about the history and the contemporary reality of hatred for Jews. It is an odd cluelessness for a man who declares himself a great friend of Israel and has three Jewish grandchildren.

After neo-Nazis in Charlottesville last year bellowed “Jews will not replace us,” Trump could not find the words to discuss and denounce their anti-Semitism then, either. He famously declared that some of them were “very fine people.” Yet the president was not alone in finding it difficult to understand or discuss anti-Semitism. The absurdity of the idea that the small numerical minority of Jews in this country either wish to, were trying to or would be able to “replace” anyone drew less attention after the Charlottesville march than the more broadly racist ideology on display there. Discussion of Confederate statues and racism against African Americans fit more easily into the conventional understanding of hate in the United States.

Anti-Semitism fosters this perplexity because it is not about skin color and because it is thought to have been dead and buried after the defeat of Nazi Germany. Perhaps that lack of understanding is why executives at Twitter did not shut down the account of the Pittsburgh suspect after he tweeted that he was not going to passively allow the Hebrew Immigrant Aid Society to “bring invaders” into this country or “watch our people get slaughtered.” The man is reported to have said that “all Jews must die” when he entered the synagogue Saturday morning.

That insertion of “all” is instructive about the conspiratorial mentality that is central to anti-Semitism. It places the shooter in a long tradition of enemies of Judaism and the Jewish people. What has bound and binds these killers together is the conviction that we Jews, though small in number, are a uniquely powerful and evil people. That fusion of power and evil was long embedded in the New Testament story of the death of Jesus, a story told and retold by all the major Christian faiths for almost 2,000 years. It was only in 1965 that the Catholic Church decided that neither the Jews of Jesus’ time nor their descendants were responsible for the death of Christ. Much Jewish blood had flowed before the church arrived at that insight.

The absolute essence, the very core of Nazi anti-Semitism, one blared on the radio and plastered on the front pages of newspapers in Hitler’s Germany, was a secularization of that old religious-based fear. In the anti-Semitic imagination of the Nazis, the Jews became a political actor called “international Jewry.”

They claimed that Jews, having seized power in the United States, Great Britain and the Soviet Union, started World War II to exterminate the German people. So, in return, Hitler announced that he was going to defend the Germans by killing Jews in a vast act of self-defense called the Final Solution of the Jewish Question in Europe. Hitler also was aiming at all Jews around the world. The Final Solution was to be global, to crush the powerful and evil conspiracy threatening Germany. That spirit of Nazism was present in Pittsburgh today.

But Hitler’s death did not end other manifestations of anti-Semitism. It lived on in the communist attacks on the conspiracy of Zionists with “American monopoly capitalists,” during the anti-cosmopolitan purges of the early 1950s; in the New Left’s denunciation of a supposedly powerful Israel working as a tool of American imperialism in the aftermath of the Six-Day War of 1967; in the Palestine Liberation Organization’s lies that Israel was an apartheid state that practiced deliberate mass murder. It lived on among the radical Islamists in Tehran, among authors of the Hamas Charter of 1988, and in the al-Qaeda killers who attacked the “alliance of Jews and crusaders” on Sept. 11, 2001. That attack fused hatred of the United States and Jews. Every single one of these forms of anti-Semitism, though on opposite ends of the political spectrum, has a conspiratorial mind set at its core that leads to the use of violence to attack the supposedly powerful Jew, indistinguishable from the supposedly evil Zionist.

Trump’s contribution to the revival of anti-Semitism in American politics lies in his penchant for conspiracy theories — evident, for example, in the disgusting closing commercial of his 2016 campaign. It included dark insinuations about global forces, with photos of prominent Jewish figures: George Soros, Lloyd Blankfein and Janet Yellen. That commercial was an unambiguous and obvious appeal to anti-Semitism. If Nazi propagandists had seen it, they might have considered it an update of some of their own posters. Through his support for conspiratorially minded right-wing media figures, Trump has lent legitimacy to a paranoid and dangerous mode of thinking. He has associated it with the presidency of the United States, when it used to be the domain of fringe racists.

The reality is that Jews are a small minority in America and an even smaller one worldwide. And though we in the United States are mostly white, most of us also know that in this overwhelmingly Christian country, we are still an enduring “other.” We know that however sophisticated and tolerant our Christian friends are — and we cherish our friendships and bonds with them — that as long as Christians read the New Testament, there will always be some — however few in number — who take its words literally and will look upon Judaism and Jews with a mixture of disdain, hatred and fear.

It is time for us all, including our political leaders, to spend some time studying where hatred for Jews comes from and how it can and must be defeated — yet again. The time is long past to end American cluelessness about the history, nature and contemporary danger of anti-Semitism.

CORRECTION: An earlier version of this article misstated one of the people pictured in President Trump’s final campaign ad in 2016. The ad featured Janet Yellen, not Ruth Bader Ginsburg.

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