"Wherever there is trouble in Europe, wherever rumours of war circulate and men's minds are distraught with fear of change and calamity, you may be sure that a hooked-nosed Rothschild is at his games somewhere near the region of the disturbances."

So said Labour Leader, the newspaper of the Independent Labour Party, back in 1891. A mere 127 years later, the current leader of the Labour Party is in trouble for apparently backing a mural that, quite literally, featured a caricature of a hook-nosed Rothschild. Safe to say, antisemitism has been around in the Labour Party for a lot longer than Jeremy Corbyn.

The mural at the centre of this latest row was painted on an East London street by American graffiti artist Mear One in 2012. It depicted a group of old white men playing monopoly on the backs of downtrodden, anonymous workers. Above them sat an all-seeing eye within a Masonic pyramid; behind was a dystopian industrial landscape with the slogan "The New World Order is the Enemy of Humanity".

This was a piece of conspiracy art in which the antisemitism wasn't even that subtle: the Jewish bankers were differentiated from the others by having bigger noses. There was consensus among Tower Hamlets politicians that it was antisemitic and should be removed. The then-Mayor, Lutfur Rahman, and Conservative group leader Peter Golds, both thought so, and they didn't usually agree on much. But Corbyn was of a different mind. He wrote on Mear One's Facebook page that the artist was "in good company", because "Rockerfeller [sic] destroyed Diego Viera’s [sic] mural because it includes a picture of Lenin.”

Corbyn now claims that he was just supporting freedom of expression and didn't really look at the mural before commenting. This is unconvincing. He must have sought out Mear One's Facebook post in order to comment there, and the most likely reason is that he was aware of the widely-reported allegations that the mural was antisemitic. The comparison to Mexican artist Diego Rivera suggests that Corbyn saw Mear One's mural in a similar radical tradition to his work.

In fact the mural is from a very different, less appealing left-wing tradition. Back when Labour Leader was writing about the Rothschilds, German Social Democrats had a name for fellow socialists, who mixed opposition to capitalism with antisemitic conspiracy theories about Jewish bankers. They called it the Socialism of Fools, because it looked like a progressive, emancipatory politics that sought to free the working masses from the exploitation of global finance, but in fact was just a socialist version of Europe's oldest and most adaptable prejudice.

The British left has never been immune to this Socialism of Fools. In 1900, the Trades Union Congress passed a resolution arguing the Second Boer War was being fought “to secure the gold fields of South Africa for cosmopolitan Jews, most of whom had no patriotism and no country.” Even the great Keir Hardie once wrote that “modern imperialism is really run by half a dozen financial houses, many of them Jews, to whom politics is a counter in the game of buying and selling securities.” This is precisely the image used in Mear One’s mural. And when a Labour government took Britain to war in Iraq in 2003, the idea that this was the result of Zionist string-pulling in Washington and London became commonplace across the left.

The Labour Party has treated cases of antisemitism amongst its members as random anomalies, as if they involve people who have wandered into the wrong party by mistake or used an unfortunate choice of words. This misses the point: the left has always had its own forms of antisemitism, well before Israel existed, and which appeal to people of a progressive mindset. Conspiratorial depictions of Zionism and obsessive hatred of Israel provide fertile soil for this current variant. It is part of a worldview that has usually been confined to the margins of the left, and tends to erupt into the mainstream at times of political unrest and uncertainty of the sort Western politics is currently experiencing.

The question now is whether Jeremy Corbyn and the Labour Party even grasp that this antisemitic political culture is active and growing within its ranks. If Corbyn genuinely didn’t understand that caricatures of big-nosed Jewish bankers in a conspiracy theory setting are antisemitic, his generic claims to always oppose antisemitism are worthless. How can he oppose something that he doesn’t understand and can’t recognise?

The alternative – that Corbyn did recognise it as antisemitic, but supported it anyway – doesn’t bear thinking about; but this is exactly what a lot of people in the Jewish community are now thinking. The time is running out for Corbyn and the Labour Party to prove them wrong.

Dave Rich is Head of Policy for the Community Security Trust and author of The Left’s Jewish Problem: Jeremy Corbyn, Israel and Anti-Semitism (Biteback, 2016).