Last week Jeremy Corbyn made history in all the wrong ways. Chief Rabbi Ephraim Mirvis attacked him for racism. Corbyn was asked thrice by interviewer Andrew Neil if the view that “Rothschild Zionists run world governments” was antisemitic. He struggled to condemn it. Both are unique moments.

When I was growing up my mother frequently told me to “be grateful daily to be Jewish in Britain”. That’s why, faced with the untrue claims that antisemitism was being honestly confronted in Labour, British Jews are still surprised by Corbynism and the Chief Rabbi was convinced he had to act.

His intervention is unprecedented since 1694 when Chief Rabbis were first elected – as is the Archbishop of Canterbury’s support for the Chief Rabbi. Corbynists claimed the Chief Rabbi was a Boris Johnson fan (but only cited the message all faith leaders send to new PMs). They insist Jews are exaggerating the problem. Yet nobody would question any other racial group – especially one with such a history. The Chief Rabbi represents the views of the majority of British Jews: 84% believe Labour is antisemitic; 87% that Corbyn is antisemitic; 93% won’t vote Labour.

Yet it was the exchange about Jewish conspiracies during the Neil interview that baffled non-Jewish people – and revealed most.

“Rothschild Zionist… world government” has a dark history as a 21st-century strain of medieval racism. Evil moneylenders with mixed loyalties inspired Edward I’s terror that culminated in the hanging of 269 innocents, then the expulsion of all Jews in 1290. “Because of their crimes and honour of Crucified Jesus, the King banishes these perfidious men,” he declared. The trope of world governments of Jewish bankers derives from Russian antisemitism which was a fetish for the last Romanovs, especially Nicholas II, expressed in the forged Protocols of Elders of Zion –later recycled with murderous vigour by Hitler.

The intervention of Ephraim Mirvis is unprecedented since 1694 when Chief Rabbis were first elected

The lacing of this chalice with anti-Zionism was the work of Stalin. Obsessed with the “mystical” supranational nature of Jewishness, Stalin initially supported the creation of Israel, rightly regarding Zionism as one of the national liberation movements heralding the end of Empire, not unlike Indian nationalism. But when Israel allied with America, he turned savagely against Jews, coining the phrase “rootless cosmopolitans”, welding Zionism on to a world Jewish conspiracy of evil bankers in which Jews were not a race but a class of bloodsucking capitalists feasting on the proletariat. In 1948 he arranged a traffic accident to kill a celebrated Soviet-Jewish actor Solomon Mikhoels. In 1952 he launched the anti-Jewish doctors’ plot as the start of a new terror.

Obviously Corbyn is no Stalin. But the mural of hooknosed Jewish bankers, alliances with antisemitic terrorists and refusal to condemn many disgusting anti-Jewish confabulations are the rotten fruit of this ideology, now oozing from Corbynists all over the internet. Corbyn himself has met with Holocaust denier Paul Eisen. Stalin refused to admit Hitler’s final solution was any different from his killing of other Soviet citizens.

Why should this Jewish problem matter to non-Jews faced with Brexit, Tories, austerity? It undermines the Labour leadership’s ability to be real progressives at all. Hostile towards Western democracies for their “imperialism”, Corbynists support dictatorships in Russia, Iran and Venezuela while claiming that what is called “antisemitism” is not anti-Jewish, merely pro-peace, pro-Palestine, anti-Israel.

That is untrue. It is fine to criticise the Israeli government without being antisemitic. British Jews support a Palestinian state alongside Israel. But Corbynists are fixated with the destruction of Israel above all other causes – and that includes a strange neglect for the plight of any other Arab peoples such as the 500,000 killed in Syria by Bashar Assad. (Emily Thornberry recently insisted on Assad’s popularity in Syria.) This is not really about Israel but a preposterous worldview that requires Jews as enemies, only making sense in a shady cavern of conspiracy theories, the stupid path that has shamed a great party.

This racist rot poisons any good Labour might do. There are flickers of hope that calm reason can work amid Twitter shrieking. Tuesday: historian Sir Richard Evans tweeted he’d vote Labour. Thursday: after lawyer Anthony Julius wrote an open letter to the New Statesman, he courageously changed his mind.

Tragically, this is not just about one man: Labour is now controlled by this thuggish camarilla while frontbench “moderates” passively enable Corbyn. But Britain’s soul is at stake: many decent Labour supporters will surely show that they are better than the racism of their shameful shameless leadership.

And I am still grateful I was born Jewish in Britain.

Simon Sebag Montefiore’s books include Stalin: The Court of the Red Tsar and Jerusalem