No political leader in Europe embodies that sentiment more than Britain’s Labour Party leader Jeremy Corbyn. He paid respects at the memorial of the Palestinian perpetrators of the 1972 Munich Olympics massacre. He objected to the destruction of a street mural depicting despotic hooknosed Jewish bankers. He participated for over a decade in the activities of a group called Deir Yassin Remembered, which was led by a Holocaust denier. He publicly defended a virulently anti-Semitic vicar named Stephen Sizer. He invited an Islamist preacher who believes Jews use gentile blood for religious reasons to tea at Parliament. And so on.

And yet he adamantly denies being an anti-Semite, on the grounds that he has devoted his life to “exposing racism in any form.”

Anti-Semitism, though, isn’t just a brand of bigotry. It’s a conspiracy theory in which Jews play the starring role in spreading evil in the world. While racists see themselves as proudly punching down, anti-Semites perceive themselves as punching up.

The Israeli writer Yossi Klein Halevi put it elegantly: “What anti-Semitism does is turn the Jews — the Jew — into the symbol of whatever a given civilization defines as its most loathsome qualities.” When you look through this dark lens, you can understand how, under Communism, the Jews were the capitalists. How under Nazism, the Jews were the race contaminators. And how for Mr. Corbyn and his ilk on the left, Israel, the Jew among the nations, is the last bastion of white, racist colonialism.

European Jews must now contend with this three-headed dragon: Physical fear of violent assault, often by young Muslim men, which leads many Jews to hide evidence of their religious identity. Moral fear of ideological vilification, mainly by the far left, which causes at least some Jews to downplay their sympathies for Israel. And political fear of resurgent fascism, which can cause some cognitive dissonance since at least some of Europe’s neo-fascists profess sympathy for Israel while expressing open hostility to Muslims.

Now these three strains of hate are beginning to show up on this side of the Atlantic.

The biggest threat is on the far right. This is the anti-Semitism of “Jews will not replace us” marchers in Charlottesville, Va., and the killer at the Tree of Life synagogue in Pittsburgh who ranted against globalists and the “kike infestation.” It is the anti-Semitism of Representative Steve King of Iowa and of alt-right Reddit boards and some of Donald Trump’s supporters.

Islamism is far less of a threat in the United States than in Europe — we do not, contrary to what the president would have you believe, have caravans of terrorists crossing our border. Still, a Muslim-American who expressed hatred of Israel shot six people, killing one of them, at the Jewish Federation of Greater Seattle, in 2006. Four Muslim men were arrested in a plot to bomb two Bronx synagogues in 2009. A Muslim convert was thwarted by the F.B.I. in his plan to blow up a Florida synagogue in 2016. Just last week, Mohamed Mohamed Abdi, a Somalian immigrant, shouted anti-Semitic slurs while trying to run down with his car people leaving a Los Angeles synagogue.