As east Aleppo falls to Bashar al-Assad’s murderous regime, the postmortem has begun. How could this sickening onslaught and its miserable human cost have been avoided? All too predictably, fingers point back at the culprits of 2013, those who prevented the west from bombing Assad’s forces. Those who backed the wars in Iraq and Libya feel tainted by the bloodshed in the calamities that followed. They accept that the killing fields of Iraq and the disintegrated state of Libya substantially weakened the moral case for western intervention. The horror of Aleppo presents the counter-argument: the cost of inaction. Those who opposed intervention are also stained with blood, goes the argument. It feels grubby to enter such a debate as Aleppo burns, but the revision of history demands a response.

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Let us be clear. Assad and Vladimir Putin are responsible for heinous crimes. Social media abounds with their apologists, those who think they are so radical, the arch-critics of western imperialism, but who are actually both hypocrites and a moral disgrace. Yesterday’s front page of the Morning Star rightly provoked revulsion when it described Aleppo’s fall as a “liberation”. When the US pummels countries with bombs, such apologists would never dream of denying civilian casualties. When Russia and its allies are responsible, they echo the language of the most ardent neocon: that the dead in Aleppo are not civilians but terrorists; that civilian deaths are either inventions or entirely the responsibility of rebel militia; that civilians are all rejoicing at their “liberation”. On Tuesday Syria’s ambassador to the United Nations, Bashar Jaafari, held up a photograph of a kindly soldier helping a woman from a truck. “This is a Syrian soldier,” he claimed. “She is a woman fleeing eastern Aleppo.” In the age of the internet, such lies are unwise. It was swiftly discovered that this was, in fact, a woman in Falluja, in neighbouring Iraq, being helped by state militia.

Falluja itself is a reminder of the moral bankruptcy of those who criticise western imperialism but apologise for Russia. They were the first to – rightly – denounce US forces who assaulted the city in 2004, using white phosphorus as they did so, a substance that can burn down to the bone. And yet they deny an even worse atrocity today, because it is not being committed by the US. The UN has received credible evidence of up to 82 Aleppo civilians being shot dead where they stood. Amnesty International speaks of “reports that civilians – including children – are being massacred in cold blood”, and that Russian-backed Syrian forces “have repeatedly displayed a callous disregard for international humanitarian law”.

Up to 100,000 civilians are said to be crammed into an area of no more than two square kilometres: Lina Shami, an activist in Aleppo, reports there are “no safe areas” and that civilians are “threatened with field executions or are dying under bombing”. Russian bombing has been indiscriminate, annihilating local infrastructure. Yes, what began as a democratic struggle in Syria more than five years ago has been sabotaged by Islamist extremists – such as Islamic State, which came in from post-invasion Iraq – but Aleppo had a democratically elected council and independent civil society, now lost.

Are those who opposed military intervention against Assad in 2013, before the west turned its firepower on his Islamist extremist opponents, to blame? Here is what the then prime minister, David Cameron, told the House of Commons in 2013 in the debate before proposed intervention. “It is not about taking sides in the Syrian conflict, it is not about invading, it is not about regime change, and it is not even about working more closely with the opposition: it is about the large-scale use of chemical weapons and our response to a war crime – nothing else.” Whatever the argument for larger-scale intervention – and why it would have been opposed, because of Iraq and Libya, because of the ascendant extremist groups in Syria’s conflicts – it wasn’t even on the table.

Neither was it proposed by the US president, Barack Obama. As the New York Times reported in August 2013, action was considered to “deter and degrade” Assad’s “ability to launch chemical weapons” – but it was explicitly not about regime change or even “forcing him to the negotiating table”.

Nothing about specific areas of Syria, nothing about the rebels. It was explicitly sold as a limited operation focused entirely on chemical weapons. Yes, there was the obvious fear that this would mean creeping direct military involvement, and there is no precedent of western military intervention in the Arab world ending in anything other than disaster. It was, nonetheless, not what was debated.

There are, however, concrete actions that the west could be undertaking now. As Mark Boothroyd from Syria Solidarity Movement UK tells me, the case for British humanitarian airdrops in Aleppo and elsewhere is overwhelming. These would show support for people who are besieged, whom Assad and his Russian backers are trying to starve out. A UN-monitored evacuation plan needs to be put in place. Russia needs to come under pressure for its criminal behaviour in Syria – with further sanctions considered.

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Syria is a reminder of the need for consistency. Those of us who passionately opposed the disastrous western wars in Iraq and Libya are not apologists for Putin or Assad. Similarly, those who denounce the opponents of western intervention should have far more humility about Iraq and Libya: the hundreds of thousands dead, the sectarian conflict, the millions displaced and traumatised, the extremist groups flourishing in the chaos. They should speak out about the west’s alliance with a head-chopping dictatorship such as Saudi Arabia, which exports extremism - including to Syria - and which is butchering Yemeni civilians with British-made bombs.

If you oppose war crimes, if you oppose the murder of innocent civilians, you should speak out about whoever is responsible. There is no contradiction in opposing the crimes of western or Russian foreign policy, or in denouncing both the bombs of Syria and of Saudi Arabia. As I say, call it consistency. Or perhaps a better word is humanity.