Something about the term “barrel bomb” fails to convey the horror of the weapon. Perhaps it is the alliteration, which has a kind of playful quality; maybe it’s because we associate barrels with beer and wine. But barrel bombs are the source of unimaginable horror and cruelty. As well as explosives, they often contain shrapnel to maximise the human carnage. Dropped from helicopters at heights that make precision targeting impossible, they are employed by the Bashar al-Assad regime, our de facto allies – let’s stop pretending otherwise – and until recently, by the Iraqi government too. In just a year, barrel bombs killed more than 6,000 civilians in Syria, nearly a third of whom were children.

But the Assad regime does not flaunt its cruelty. It does not make videos with Hollywood effects – slo-mo, closeups, haunting music, the aftermath in high definition. Instead, it adopts the same regretful tone of western powers, like when the US dropped flesh-burning white phosphorus over Falluja. We regret any civilian casualties (or “collateral damage”, as the west prefers). We do not target civilians, unlike our opponents – and so on. The scale of death may be far greater, but the claimed intentions are different: unlike our opponents, we do not aim to kill civilians, they say, so we retain our moral superiority. Above all, the Assad regime does not execute white westerners and film it. Islamic State (Isis) is now the iconic demon, the stuff of nightmares – which is exactly what it wants, of course.

If we can provide context for the rise of Nazism, why not elsewhere?

At times of war, failing to participate with sufficient zealotry in the vilification of the current public enemy number one is treated as apologising for evil, or even as near-treachery. In the summer of 2013, that applied to the Assad regime after it allegedly gassed hundreds of innocent civilians to death. Isis has now supplanted it: an Orwellian “we have always been at war with …” mentality kicks in. Nobody should be under any illusion that Isis militants are not barbaric murderers who need to be defeated, even if we differ on how such a defeat will be achieved. But it is now the fashion to grant them a unique evil, a nightmarish mystique they crave: both because it allows them to rout their enemies, who are so terrified they flee rather than fight, and because it bolsters their reputation among sympathisers, helping to win recruits.

Here’s an example from a recent column in a British newspaper: “In Isis we are observing a level of atrocity towards mankind that, post-Nazism, we hoped we would never again witness.” Really? What about Pol Pot and his killing fields? The mass murder of a million communists in Indonesia in the 1960s, which turned rivers red with blood? The war in the Democratic Republic of the Congo, which killed up to 6 million people and featured mass cannibalism? The US carpet bombing of Vietnam, Laos and Cambodia? Isis decapitates its victims, just like our friends the Saudis – but again, they kill alleged “sorcerers” off-camera. Herein lies the danger: it is in the interests of both western powers and Isis to grant this bunch of terrorists an almost supernatural horror.

Mohammed Emwazi was recently named but he is still widely known as “Jihadi John”; only now he is approaching Osama Bin Laden levels of fame. This former Westminster University student must relish being transformed into the west’s demon icon, a notoriety that will be matched by veneration among Isis true believers. In such an atmosphere, a level of understanding that goes beyond “they were infected with a poisonous ideology” is treated as justification. I do not know how Emwazi was radicalised, and anybody who says definitively that they do is a fool or a fraudster, but it certainly makes sense to examine every possible factor. We are unable to do so, because such an enterprise risks being portrayed as aiding Isis, ironically by those who are doing just that.

Is examining the role of, say, Versailles and economic crisis in the rise of Nazism making excuses for it? If we provide such context for the most barbarous ideology in human history, why not elsewhere?

What makes this all the more cynical is the west’s inconsistent – shall we say – attitude to jihadism. Who did western powers help to back, bankroll, train and arm in Afghanistan in the 1980s, but the first major international group of jihadis who later exported their terror? Which fundamentalist Middle Eastern dictatorships do we arm and support, even though their kingdoms all too often export extreme ideologies as well as funding and arms to jihadis? We knew that jihadis were fighting against Colonel Gaddafi in 2011, and as we bombed Libya, we were their de facto allies. Thanks to western intervention, large chunks of Libya are now under their sway.

And let’s drop the pretence that the west did not effectively back jihadis in Syria either. When Theo Padnos was released after being held in captivity by the al-Nusra Front, he trashed the myth that our “moderate” Free Syrian Army allies were separate from jihadis, who twice returned him to his captors. Newspaper articles from 2012 are reminders that we knew aid from our Saudi and Qatari allies was ending up with jihadis.

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Some will call the west’s dizzying shifts in foreign policy – Gaddafi from foe to friend to foe, Iran from arch-nemesis to de facto ally, and so on – realpolitik, the sort of pragmatism necessary in a complex world. Easy for the likes of me to carp. Perhaps, but 14 years of the “war on terror” has culminated in fundamentalist groups who are more extreme and powerful than ever.

A rational conversation about causes and possible solutions is all but impossible. We must simply say that Isis is evil in its rawest form, apparently more evil than anything that has gone before, and leave it at that. Bombs and prisons: that’s the only legitimate response, and anyone who says otherwise is an apologist, a traitor or both. And so the symbiotic relationship between western power and jihadism persists.