When the Chilcot report is finally released – perhaps at some point in the 24th century – it will be a reminder of the greatest western foreign policy catastrophe of our time. Hundreds of thousands dead, many more maimed; millions displaced; the torture cells; the dropping of white phosphorus on Fallujah; the unleashing of grotesque sectarian conflict; the rise of fundamentalist extremists, culminating in the nightmare of Isis.

But it was those who criticised the war whose careers suffered: the resignations came from the likes of Robin Cook and the heads of the BBC, not those who prosecuted the war. Tony Blair’s career flourished, allowing him to work for murderous despots with no criticism – with honourable exceptions – from his own supporters.

That’s why Jeremy Corbyn, the possible next Labour leader, is right to pledge to apologise for the Iraq war. Only by doing so can lessons be learned and future catastrophes averted. He is similarly right to call for Isis to be tackled by challenging sickening dictatorships such as that of Saudi Arabia – which beheads people for being gay and which is armed to the teeth by our countries – which are the heartlands of extremism. These points are not made because the west is the cause of all global injustice – it is certainly not – but in recognition of the fact that we have most influence over the actions of our own governments.

But it is for his stance on foreign-policy issues that Corbyn is now under particular fire. The Tories are currently assembling the mother of all attack dossiers to deploy against Corbyn should he triumph in the Labour leadership contest, and foreign policy will be at the absolute core of it. He will be portrayed as a Britain-hater who associates with extremists who wish the west ill.

One of Corbyn’s great foreign-policy passions over the years has been Palestine. And it is here he is accused of having shared platforms or met with antisemites.

As someone who spends a lot of time speaking at events ranging from conferences to protests – though nowhere near as much as Corbyn has over several decades – I know how absurdly impractical it would be to research the backgrounds of every other participant. Corbyn himself abhors antisemitism, and anti-racism is central to his political DNA. The problem is this. Most people in the movement for Palestinian justice are motivated by well, justice, for a people deprived of that basic right: national self-determination. But there is a minority who are antisemites. That means those – like Corbyn, or me, for that matter – who are active in the cause for Palestinian justice are at risk of inadvertently associating with antisemites. These antisemites have to be unequivocally challenged and driven out.

One response to the current onslaught against Jeremy Corbyn is to become defensive. A far better response is to accept that antisemitism lurks in progressive movements and make sure that it is recognised, routed out and defeated.