Consider the following: Labour has had to suspend 18 members, including one MP and a former mayor of London, because of their allegedly racist displays. Everywhere, “Zionist” and “Zio” are used to define Jews, and non-Jews who question left orthodoxy, as “the other”: barely human monsters, who must be cast from the bounds of leftish society. The leader of the Labour party has defended supporters of every variety of ancient prejudice: the Palestinian activist who revived the medieval libel that Jews used the blood of Christian children to make bread; the Anglican vicar who promoted the views of modern neo-Nazis that the Jewish conspiracy was now so malign and supernaturally powerful it was responsible for 9/11. After reviving old prejudices, Labour members adopt new ones just for fun. Jews were the chief financiers of the slave trade, they say as they repeat a fantasy promoted by the US race-huckster Louis Farrakhan. Jews collaborated with Hitler, they continue as they repeat the fantasies of 20th-century Marxist‑Leninists.

To get a bearing, imagine that Theresa May and leading members of the cabinet had endorsed the supporters and ideology of the Ku Klux Klan or Britain First, and then rewarded the chairwoman of a supposedly independent inquiry into rightwing racism with a peerage.

In the 70s, there was at least an awareness of the danger of slipping into reactionary politics

How a party that was once proud of its anti-fascist traditions became the natural home for creeps, cranks and conspiracists is the subject of Dave Rich’s authoritative history of left antisemitism, which is all the more powerful for its moral and intellectual scrupulousness. Rich knows as well as anyone there are many lefts, not one homogeneous movement. Unfortunately for Britain, representatives of the darkest left factions control Labour and much of the trade union movement, and dominate the intelligentsia. On the rare occasions when they are challenged, they say “I am not antisemitic, I am just anti-Zionist” – an excuse that mirrors the protestations of the Daily Mail that it is not against ethnic minorities, just anti-PC. Rich uses it as key to understanding the strange history of how a leftwing movement many Jews once supported became an unsafe space.

Zionism is the nationalist belief that the Jews should have a homeland. Once a controversial idea among European Jews, it turned into common sense as fascism grew. What, in these circumstances, does anti-Zionism mean? From the 1970s on, opponents of Israel had to decide whether anti-Zionism meant a fulfilment of Palestinian national rights via a two-state solution which recognised that Palestine was the focus of competing Jewish and Arab nationalisms, or whether it required them to support a war to the death, which would lead to an ethnically and (with the rise of Sunni fundamentalism) confessionally pure state. The question is rarely asked, let alone answered. Today’s Palestine solidarity movements lack the guts to say openly that they want an end to Israel, and instead argue for the right of Palestinians to return to Israel, which would destroy it as a Jewish state.

I suppose it is possible to agitate for the destruction of the world’s only Jewish country without being a racist. Rich shows in the 1970s there was at least an awareness of the danger of slipping into reactionary politics. The far left tried to ban Jews speaking at universities and supported violence, but it very rarely talked in Hitlerian tones of a vast Jewish conspiracy.

The Observer view on Labour’s antisemitism crisis | Observer editorial Read more

Anti-fascism died when Islamist utopianism annihilated socialist utopianism. At a pro-Palestinian rally in the 20th century, you would hear dreams of a future where the Arab and Jewish working classes would unite in a common homeland. By contrast, at a pro-Palestinian rally led by Corbyn in 2002, supporters of the Muslim Brotherhood handed out newspapers instructing marchers that man was on Earth to serve God, and Muslims and non-Muslims could not be treated equally in an Islamic state. All of a sudden, and without anything resembling a debate, the loudest voices in the British and world left were on the side of men whose prejudices, not only against Jews, but against women, homosexuals, secular societies, and human rights, combined the worst theology of the seventh century with the worst ideology of the 20th.

If Rich has a fault, it is that as a rational historian, he cannot speculate on the psychological appeal of left antisemitism. Novelists would notice the attraction of an authorised racism to leftists, who in other respects are highly constrained in what they can say by speech codes. They would find that a prejudice that endorsed fascism as well as 2,000 years of Christian polemics and persecutions fitted far-left political beliefs rather well. It is not a large step, after all, to go from saying that democracy is a swindle perpetrated by the “neoliberal” world order, to saying that the swindlers are the “Rothschilds”, the “Zionists” the – oh, let’s just spit it out – the Jews.

The damage that the embrace of the politics of the radical and religious right has done to progressive causes ought to be evident enough one year into Corbyn’s leadership. The damage that will be done if it is not stopped is, in my view, incalculable.

The Left’s Jewish Problem: Jeremy Corbyn, Israel and Anti‑Semitism is published by Biteback (£12.99). Click here to buy it for £10.50