Every time a massacre happens, we say "never again". Now, "never again" is happening in Aleppo, as I write.

The government's operation to recapture the rebel-held eastern part of the city is in its final stages and reports of atrocities are coming in.

The Syrian army is reportedly going from house to house and executing residents on the spot. At least 82 civilians, including women and children, were shot on Monday, according to a spokesperson for the UN High Commissioner for Human Rights.

Residents fear summary executions, forced disappearances, torture, and rape – a grim litany of war crimes. Meanwhile, the Syrian regime and the Russian government are ignoring calls for the safe passage of civilians from war-torn Aleppo, as more than 100,000 terrified people are still trapped inside the city. 

If this story sounds familiar, that is because we have heard it before. We have seen it on the killing fields of Cambodia, the ghost towns of Iraqi Kurdistan poisoned by chemical weapons, in the faces of machete-wielding Rwandans, the sieges of Sarajevo and Srebrenica, and the desert death camps of Darfur.

"Never again," the world pledged in the wake of these atrocities. And yet the same horrors are now being inflicted on the people of Aleppo and we are reacting with much the same carelessness.

We cannot say we did not know. We knew then and we know now. We can watch in real time how terrified residents post their final "goodbyes” on social media. The Syrian Civil Defence, also known as the White Helmets, who act as first responders, issued a desperate final plea to the world: "The bombs are falling as we write this.

For years our humanitarian volunteers have worked to save the lives of our people in Aleppo: operating in underground hospitals, rescuing entire families buried under rubble and risking our lives to document what the daily war crimes committed by Assad regime and its ally Russia. We can do no more.”

The UN, paralysed by Russia’s and China’s veto in the Security Council, has long been reduced to issuing empty mantras ranging from being “very concerned” to “deeply concerned” and, lately, “gravely concerned”.

But their condemnation of crimes committed against innocent civilians does not feed the children on the streets of Aleppo.

It does not keep warm the elderly, or help the men pulling women from the rubbles of the ruins of their city.

Five years into the civil war, Syria has turned into the greatest humanitarian catastrophe of our time. All red lines have been crossed.

One of the greatest failures of our political leadership has been to allow Syria and Russia to dominate the narrative around Aleppo.

Everyone is now a terrorist. Every hospital bombed a secret weapons storage. Every use of chemical weapons a false flag operation by the rebels.

Of course you can take the position that Syria is not our war. After all, the public appetite to involve ourselves in conflicts in far-flung places is at a decades-long low.

But do not fool yourself into thinking that the consequences of a war of such scale can be contained to the borders of its country.

Aleppo will become the ultimate symbol of anger and disillusion. It will drive more young men into the arms of Islamic State and other terrorist groups and it will bring more refugees to Europe’s shores. 

We did not learn the right lessons from decades of war. Non-intervention is not the answer to the failures of intervention.

The truth is that full-scale interventions as seen as in Afghanistan and Iraq are difficult. And so are partial interventions as we have experienced in Libya.

But if anything, Syria has shown that consequences of inaction must not be ignored or forgotten, for they can sometimes have repercussions even more devastating than the choice to take action. 

It is too late for the people of Aleppo. They will follow the restless souls of Cambodia, Rwanda, Bosnia, and Darfur in a long line of victims abandoned by the international community.

At least we have another opportunity to reflect on how we want to respond to mass atrocities in the future. 

Julie Lenarz is executive director of the Human Security Centre