Why would the leader of a mainstream political party declare that he is opposed to antisemitism? The answer, in the case of Jeremy Corbyn, is that otherwise it would be impossible to tell.

Labour leaders such as Hugh Gaitskell, Harold Wilson, Tony Blair and Gordon Brown had deep understanding of Jewish values and instinctive sympathy with Israel.

By contrast, Corbyn has a long and documented history of allying with bigots.

For years he supported a group called Deir Yassin Remembered whose founder, Paul Eisen, is a declared Holocaust denier. Corbyn was photographed at one of the group’s events in 2013.

When Corbyn emerged as the surprise frontrunner for the Labour leadership in 2015, anyone familiar with his record could have predicted that stuff like this would come out. So it did.

Amid the resulting furore, Corbyn’s campaign approached the JC to suggest an interview. Stephen Pollard, the editor, agreed and asked me to do the interview. On learning who he’d be sitting down with, Corbyn pulled out and has not been in touch with the JC since. Though the option of communicating with the JC’s readers remains open to him, he doesn’t get to choose the questioner any more than he gets to choose the questions.

I make no claims to omniscience but if we’d had that chat in 2015, Corbyn might have been forewarned of the mess he’s now in.

His response to the revelations of his Facebook group memberships and his endorsement of an antisemitic mural is ever the same: he didn’t notice anything wrong, and he expresses “sincere regret” at not having looked more closely.

The response is feeble. On his own admission, Corbyn looked at a photograph of a mural whose iconography recalls hoary conspiracy theories about Jewish banking cartels, and he saw nothing amiss.

As he cannot grasp the moral import of his fact, there is no point in pressing him to say sorry. An apology that isn’t intended and has no costs is meaningless.

Corbyn is not an adolescent of inchoate views: he’s 68 and appears never to have changed his mind on anything. He is one of those politicians whose views you can predict with unerring accuracy on absolutely everything. All I can reasonably do is try to delineate how such intellectually crude and disreputable notions have taken hold on the left and, through Corbyn, insinuated themselves in Britain’s main opposition party.

It’s purportedly about Israel; but it’s not really about Israel. For a generation of left-wingers, such as Ian Mikardo and Richard Crossman, who served as Labour MPs at the time of Israel’s founding, the Jewish state was a cause to be celebrated.

That progressive instinct was right. To establish a democratic state, with sexual equality and separation of civic and religious authority, and while in state of siege, was a huge achievement.

For Israel to have done this while facing down the extremist wing of the Zionist movement was testament to the strength of its liberal ideals. When there is eventually a two-state solution between a sovereign Palestine and a secure Israel, that too will be a realisation of the pluralism of the Jewish national cause.

But around the time of the 1968 revolts, the radical left adopted the disastrous conceit that Israel was a colonialist enterprise. And this is where the whole fateful turn of the left against Zion has merged with a much older and sinister misapprehension.

There is a Marxist literature, very short on analytical rigour, that depicts the continued existence of the Jewish people as somehow a historical mistake.

The main work in this genre is titled The Jewish Question: A Marxist Interpretation by a Belgian Trotskyist called Abram Leon, who perished at Auschwitz in 1944. Leon maintains that the Jewish “problem” will be solved by the disappearance of national differences within a socialist revolution. He writes: “Today national-cultural and linguistic antagonisms are only manifestations of the economic antagonism created by capitalism. With the disappearance of capitalism, the national problem will lose all acuteness.”

Even supposing this vision were desirable, it’s fanciful. But it’s a staple of far-left groups who care nothing for the stubborn attachments of real people to cultural, linguistic and national traditions, and who know nothing of Jewish history.

I don’t know if Corbyn has read this but I’m certain he’s familiar with its thesis, because it’s a standard text in the circles he’s been mixing in throughout his political life.

Once left-wingers succumb to the fallacy that Israel, rather than being a flawed democracy with a just national claim, is an oppressive colonialist state, they misunderstand far more than just the geopolitics. They also fail to grasp the connection of modern Jews with the Jewish state, and the moral imperative of ensuring that Jews must have a place of refuge.

And because, in a peculiar far-left heresy, Jewish history is destined to come to an end, there needs to be an explanation for why it hasn’t done so. The answer that some on the far left come to, I fear, lies in those grotesque cartoons of Jewish bankers. There is a thin line between modern anti-Zionism and antisemitism, and the reason the boundary is so porous is that these do have common ideological roots.

I’m sure Corbyn is sincere in his belief that he is a principled opponent of antisemitism. It just happens not to be true.

There are leftists from the generation of 1968, like Joshka Fischer — who went on to be German foreign minister 30 years later — who realised the descent of their comrades into traditional antisemitism and recoiled from it. Corbyn doesn’t even recognise antisemitism when he is literally looking straight at it.

That is why he has brought a great political party to a state of obloquy, while erstwhile supporters look on in despair.

Oliver Kamm writes for The Times