When I sat GCSE History in the summer of 1990, a few weeks before the first Gulf war, there was a staple question about the treaty of Versailles: did it cause the second world war? There was a model answer: yes and no. On the one hand; on the other hand. Georges Clemenceau’s vengeful myopia and exorbitant reparations weighed against the Depression and deeper roots of German fascism.

The question is still on the syllabus, but now sits alongside a module on “conflict and tension 1990-2009”, which makes me feel old. Is this history already? I don’t know how thoroughly today’s teenagers are expected to excavate the roots of jihadi terrorism. I do know that if the question is whether western interventions caused mass murder on the streets of Paris last week, to answer yes is historical and moral stupidity without rebuttal by the case for no – just as it would have been facile to have said Versailles spawned Hitler, and put the pen down.

This is not an argument for treating Islamic State as a threat to European democracy equivalent to the Third Reich, nor a denigration of everyone who counsels military caution as modern-day Chamberlains. Isis is the most remorselessly slaughter-hungry creed to stalk the continent since the 1930s; but it is also a distinct phenomenon craving legitimation as a state engaged in conventional war, which it is not.

The charge sheet against western policy dating back a generation is easily drafted. It takes moments to weave a tale of counterproductive geopolitical vandalism, starting from US support for the mujahideen against the Soviets in Afghanistan, via the chaos of post-Saddam Iraq, pausing to condemn blind eyes turned and arms sold to Saudi Arabia, whence the theology of infidel-murder pullulates.

But to stop there is lazy. Worse, it takes an effort of analytical obtuseness to make aggressive western governments the initiating agent of all that is sinister, void of good intent or positive consequence, and thus explain jihadism as a symptom, with the CIA and Tony Blair as the virus. As if the Taliban should have been left to rule Afghanistan; as if the insurgency against allied forces in Iraq were a national liberation front akin to anti-colonial movements against the British Empire; as if Isis presented negotiable terms of secular grievance that can be settled at a peace conference; as if the rhetoric against “Zionist-Crusaders”, the genocide of Yazidis and the systematic enslavement of women were all logical extrapolations from a dodgy strategy cooked up in the Pentagon: extreme, yes, but explicable by cross-reference to prior western offences.

Yet this join-the-interventionist-dots view of terrorism’s genesis holds sway in the office of the leader of the Labour party. It is not endorsed by many MPs, which is why Jeremy Corbyn was verbally assailed by irate colleagues in a meeting of his parliamentary party on Monday night. The trigger for their fury was a round of TV interviews Corbyn had given resisting the use of drones to kill British jihadis in Syria, refusing to countenance any military action against terror groups and recoiling from a policy of permitting security services shoot-to-kill licence, anticipating a scenario like the one that has traumatised Paris. Veterans of such meetings say they have never seen a leader so isolated.

But the most revealing of Corbyn’s public statements was not about policy. It was his response when the BBC’s Laura Kuenssberg read a quote from an article by the Stop the War Coalition, the pressure group of which the Labour leader was until recently chair, describing the Paris attack as “reaping the whirlwind of western support for extremist violence”. Corbyn hedged: “I wouldn’t use that language.” Clearly not now that he has graduated from speaking as a backbencher at fringe rallies to addressing the nation as a potential prime minister. He did, however, advance the same argument with different language. Past interventions had “unleashed” terrorist forces, making the current situation one “we have created”.

If Corbyn was spouting incoherent gibberish, the episode would be unremarkable except as a sign of Labour’s hastening march into irrelevance. What makes it insidious is the semi-coherence, the fluency of his ellipses and the cold diffidence, mingled with didactic vanity, that seemed to urge his audience to get beyond the banal horror of the headlines, to reach the deeper insight available to those, like himself, who have been warning about interventionist folly (he reminded us) since 2001. He did not excuse the murderers. “Obviously, absolutely, blame those that did it. Absolutely, obviously Isil are totally wrong,” he said, but with a hint of impatience, making the ethical distinction between terrorist and target sound like a caveat to the more sophisticated point he was getting at.

This undercurrent of moral relativism contaminates the valid points in Corbyn’s argument. He is right that it is still unclear how British airstrikes in Syria would make a practical difference against terrorism. The memory of Jean Charles de Menezes, mistakenly gunned down by police in 2005, is reason to weigh gravely the implications of authorising a shoot-to-kill policy. Justice would have been better served if Mohammed “Jihadi John” Emwazi had been put on trial. But in the absence of a friendly constable to collar the villain in Syria, a choice had to be made about whether a drone strike was the best expedient alternative. Yes, a political settlement to the Syrian war is needed. Does Corbyn think this has hitherto escaped policymakers’ notice?

The hardest part of leadership is judging how far to stray from what is ideal for the sake of what is necessary. In making that calculation, the analysis of past policy failings is useful if it informs a prescription for what might work instead. What is the better use of western resources and power to defeat Isis, as a military force abroad and an ideology that recruits in our cities?

That goal will not be met by aligning a critique of British policy with the selective history that adorns jihadi propaganda. The solution is not an atonement list of supposed western aggressions in Muslim lands, maybe adding a few platitudes about diplomacy and the undesirability of war, then putting the pen down: all on the one hand, none on the other. Yet this is the Labour leader’s answer after so much study: some mangled history without a conclusion, half an argument, the sound of one hand wringing.