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Jeremy Corbyn has “been on a journey,” said Emily Thornberry, the shadow foreign secretary, on The Andrew Marr Show on Sunday. Such, at least, was her indulgent interpretation on being shown clips from 2011 of the Labour leader denouncing Nato as a “danger to the world.”

If Corbyn had adjusted his views to the realities of international diplomacy, it would be to his credit. But his record on security issues includes positions that are not only tendentious but repugnant. Corbyn’s supporters persistently claim that he’s unfairly treated by the media. In reality, he’s had an easy ride, especially on foreign and defence policy. Less than 20 years ago, Nato took military action against the genocidal regime of Slobodan Milosevic in Serbia; Corbyn’s response was not just to oppose this campaign but to deny outright Milosevic’s war crimes. Those crimes are documented facts, not matters of opinion.

Collective security is integral to Labour’s traditions. Among the party’s historic achievements is the Attlee government’s role in the creation of Nato in 1949, which tied the defence of western Europe to that of the United States. All postwar British governments have treated Nato membership as the bedrock of defence policy but there has been a persistent dissenting minority opinion within the Labour Party. Robin Cook, who served as foreign secretary after Labour won the 1997 election, is a case in point. In 1978, he co-authored a Fabian pamphlet that argued for “some form of partial disengagement from Nato.” But after that, Cook genuinely did go on a journey. In office, he helped commit Nato to its campaign in Serbia. On 24th March 1999, Nato aircraft began bombing Serb air defence and communications systems. The aim was to stop Milosevic’s assault on the Albanian population of Kosovo.

The Kosovo campaign was just and limited. Unlike the Iraq war, which prompted Cook to resign from Tony Blair’s government in 2003, Nato’s use of airpower in Kosovo had no aim of regime change. The war wasn’t even fought for Kosovan independence. It had the specific goal of preserving a threatened Muslim population from destruction. Milosevic had ignored diplomatic efforts to bring peace to the province of Kosovo, including a ceasefire agreement in October 1998 and the Rambouillet peace accords in February 1999 that sought to restore autonomy to Kosovo. Serb forces deployed increasingly brutal methods to drive Kosovan Albanians from their homes. It took 78 days of Nato’s air campaign, and nearly 40,000 combat sorties, before Milosevic yielded. There was no other way of getting him to abandon his campaign of ethnic cleansing.

Five years after the Kosovo war, in 2004, Corbyn signed an early day motion in the House of Commons commending a column by the campaigning journalist John Pilger in the New Statesman. The motion repeated many false assertions and “congratulate[d] John Pilger on his expose of the fraudulent justifications for intervening in a ‘genocide’ that never really existed in Kosovo.”

Pilger has long engaged in a fantastical scheme of historical revisionism in which the murderous xenophobic dullard Slobodan Milosevic is depicted as a victim of US imperialism. Pilger’s polemical methods are crude and include continually misrepresenting comments made by Tony Blair, US diplomats and Nato spokespeople to make it appear they were exaggerating the number of Kosovan victims. Last year Pilger even claimed, totally falsely, that the UN war crimes tribunal for the former Yugoslavia had exonerated Milosevic. I have challenged Pilger to debate this nonsense with me anywhere except the fake news channel Russia Today where he vented it. I’m still waiting.

This is the journalist, and that is the line, that Corbyn backed in 2004 when he was an obscure backbencher. Let me spell this out: it was possible (though in my view mistaken) to oppose Nato’s campaign on grounds of risks to civilians and the cohesion of the international order in the absence of explicit UN authorisation. It is also possible to take such a strict interpretation of the requirement to prove genocidal intent as to be wary of applying it to Milosevic’s campaign. It is not possible—in the sense of not legitimate, not accurate and not reputable—to rubbish the sufferings of Milosevic’s victims.

Talking of a genocide that “never really existed” is—let’s be clear—a reference to mass expulsions and slaughter. That campaign by Milosevic started long before Nato intervened: his forces had already driven 300,000 Kosovans from their homes, killed almost 2,000 people and razed hundreds of villages by March 1999. He increased the tempo of these persecutions as Nato took action.

Milosevic was the agent of systematic terror on ethnic grounds, in which thousands of Kosovans perished and 1.3m were displaced. In addition, Nato bombs accidentally killed around 500 Serb and Kosovan civilians. Pilger to this day refers (as did the early day motion) to a final body count of 2,788 victims of the Kosovo war, as if Nato is deliberately overstating the death toll. The final figure for those killed or missing is in fact 13,517: in a contemptible piece of sophistry, Pilger insinuates that if the bodies haven’t been found then the victims can’t be counted as dead.

I would like Corbyn to explain why, under the guise of a peace campaigner, he serves as apologist for the worst political leader in Europe since 1945. Perhaps (for he won’t agree to be interviewed by me) my colleagues in the press will ask him. It’s long past time he was made accountable.