The alliance between the white far left and the Islamist right is a dirty secret in plain sight. Few can bear to look at it. None of the books and documentaries on Corbyn’s takeover of the Labour party asked, even in passing, how people who professed to be socialists and feminists, found themselves promoting theocrats and misogynists. I have no doubt that ‘serious’ scholars will be as negligent when they come to write their accounts. In supposedly stable Britain, there is a psychological aversion to admitting that the dark corners of modern history can be the best place to find the roots of current crises.

However much respectable writers hate to admit it, you cannot understand Brexit without understanding the rise of Ukip from its beginnings on the cranky fringe of James Goldsmith’s Referendum Party. We may one day explain the rise of anti-Muslim bigotry by looking at the shift of today’s Ukip supporters from hatred of the European Union to hatred of Muslims. What applies to the far right applies to the far left: without understanding its toleration of religious extremism, little about modern Labour politics makes sense.

If Labour wins the next election, Emily Thornberry talking up Assad's popularity and Corbyn’s paid work for Iranian state TV will be mere appetisers. The British government will sympathise with, and on occasion ally with, Iran, Russia and whatever Assadist terror state rules over the remains of Syria. The Foreign Office will not just support a free Palestine, as any decent person should, but an unfree Hamas-controlled Palestine with all the Islamist restrictions on freedom of speech and conscience and the rights of women and gays.

If Labour does not win the next election, the far left’s alliance with theocrats will have had its effect. The reasonable fear that Labour sympathises with terrorists will have driven away voters who might have given it power.

Either way, shabby decisions made by neglected men and women at the turn of the century will have consequences far beyond the imagination of their contemporaries, who dismissed them as curiosities, assuming that is, that they noticed them at all.

W.B. Yeats said of meeting the leaders of the Irish Republican Brotherhood before they imprinted their name on history with the 1916 Easter Rising he:

thought before I had done

Of a mocking tale or a gibe

To please a companion

Around the fire at the club

If you had bumped into Jeremy Corbyn, George Galloway, Seumas Milne, Andrew Murray or John McDonnell in 2000, you in turn would have had material for mocking tales of weird no-hopers. The political far left at that time was tiny. The Campaign group of MPs struggled to attract more than a dozen members. I doubt the hardline faction from the old Communist party, the various successor groups to Militant, and the Socialist Workers Party could muster 10,000 members and fellow travellers between them. With communism dead, and globalisation roaring ahead they were living fossils: the fluke survivors of the mass extinction of the rest of their socialist species.

In 2002, Jeremy Corbyn attended a rally for Palestine in Trafalgar Square. Nothing unusual in that: many on the centre-left had marched for Palestine for years. But 2002 was different. They were no longer marching for the Palestinian Liberation, Organisation whose secular constitution connected it to the western left. In 2002, however, they marched alongside Islamists who believed apostasy from Islam is either “a religious offence punishable by death” or, at least, “an act of mutiny or treason, that is punishable”.

Even before the great protest against the Iraq war of 2003, the combination of the far left and the religious right could get 100,000 on to the streets. Imagine that: 100,000 people willing to listen to your speeches, wave your placards, vote for you in elections and give you an energy your moribund movement thought it had lost. Radical Islam was crack cocaine for the old left. All Islamists asked in return for the fix invigorating support was that the left ignored and excused religious fanaticism, however violently it manifested itself. They were happy to go further and merge the rump of the old socialist movement with the Islamist right.

In the 1990s, the Socialist Workers Party could see Islamism as a resource a wily left might exploit and direct. Islamists ‘could be tapped for progressive purposes providing a lead came from a rising level of workers’ struggle,' one of its ideologues opined in 1994. By the millennium it was impossible to separate the tail from the dog or decide who was leading whom. Radical Islamists killed Americans and hated Western democracy, just as the old revolutionary left had done. All socialists had to to do was forget Islamists wanted to create a clerical-fascist dystopia, and they might be comrades. The far left willingly fell into amnesia.

I experienced the shift when the Palestine Solidarity Committee called and asked me to become a patron. I had not yet realised the modern left required you to think about Israel from the moment you woke up to the moment you went to sleep, and wondered why they wanted me. ‘Because of your name”’. Ah right, I had a Jewish name it would be useful to stick on the letterhead to reassure those who worried that they were straying into racism.

I said that to my mind you had to combine a campaign for an end to the Israeli occupation of the West Bank with principled opposition to Hamas. It was all very well to announce your support for ‘the Palestinians’. They were not a homogenous bloc, but divided between too warring parties. Surely you had to decide which side you were on.

‘Do you condemn Hamas’,’ I asked.

‘We don’t think it’s our business to tell Palestinians what to think.’

‘That’s funny,’ I thought, as I turned down the offer, ‘you seem very keen on telling everyone else what to think.’

Today’s grotesque spectacles flow from the failure to take a principled stand by yesterday’s men and women. Corbyn, McDonnell, the Palestinian campaigners and all who went along with them would not draw lines. No one has the right to be surprised that anti-Semitism has exploded on the left, when its leaders did not protest against Hezbollah and Hamas quoting Nazi fantasies and the Protocols of the Elders of Zion. No one has the right to be surprised by the left’s sexism, when its leaders went along with clerics who treated women as second class citizens.

I’ve covered the flips from the far left to religious right and back again, and am glad to be joined in the task by Dave Rich, whose essential exploration of the dark side of politics, The Left’s Jewish Problem, has just been updated. Rich marshals his evidence with patience and rigour, and argues that the Islamist tail has wagged the old leftist dog for more than a decade. And so it has.

Yet in the past two years the white far left has taken sides. The leaders of the far left are for the Shia forces in Islam’s civil war, even though the majority of Britain’s Muslims are Sunnis. When Assad gases Syrian Sunnis, white leftists back Putin’s propaganda that mass poisoning of civilians could be fake news. Corbyn took the money of Assad and Putin’s allies in Tehran and has shown no willingness to protect the murdered people of Aleppo.

To date, there is no sign of British Sunni organisations realising what their friends have done. It is sufficient that Labour politicians hate Israel and ‘Zionists’ with the required ferocity. The disconnect will not last forever, and one day I expect to see this shameless and witless clique learn that, if they ride the sectarian tiger, they must expect to be eaten.