Aleppo is falling.

Syrian regime forces, backed by Iran and Russian airstrikes, are on the verge of retaking the final remnants of the rebel-held eastern portion of the city, where tens of thousands of civilians are trapped with no way of escape. A ceasefire deal, brokered by Russia and Turkey on Tuesday, was supposed to let civilians and rebels withdraw — but the Assad regime appears to have rejected it. His troops are fighting rebels in vicious block-to-block combat.

As regime forces have entered rebel territory, reports of mass killings have stunned the world. The United Nations human rights office reports one instance of 82 civilians being executed on the spot by Bashar al-Assad’s forces. Another source in the city, speaking to the Daily Beast, said that he knows of 20 women who killed themselves to avoid being raped by Assad’s forces.

Citizens, journalists, and activists trapped in the city are facing their death — openly, on public media like Twitter:

My name is Bana, I'm 7 years old. I am talking to the world now live from East #Aleppo. This is my last moment to either live or die. - Bana — Bana Alabed (@AlabedBana) December 13, 2016

My dad is injured now. I am crying.-Bana #Aleppo — Bana Alabed (@AlabedBana) December 12, 2016

Oh God .. wake us from this nightmare ..#Aleppo is being erased & more than 50.000 are under risk of being killed.

Mostly women & child. — Zouhir_AlShimale (@ZouhirAlShimale) December 12, 2016

There is no total number of casualties in Besieged #Aleppo today, all streets & destroyed buildings are full with dead bodies. It's hell. — The White Helmets (@SyriaCivilDef) December 13, 2016

Last message from this old man in Aleppo: 'We don't have anything left. Oh Arabs, where are you? We are being slaughtered. Safe us.' pic.twitter.com/E1sws1s2Y8 — Sakir Khader (@sakirkhader) December 12, 2016

What is happening right now is a slaughter, being documented in real time and beamed around the world. We will not know exactly how many were killed and tortured and raped for some time, and maybe not ever.

But we will know that it happened — and that neither the US nor the rest of the world did anything meaningful to stop it.

How we got to this disaster

Aleppo used to be a lovely place.

A city located in northwest Syria near the Turkish border, it’s the capital of the eponymous Aleppo province. Prior to the war, it was Syria’s most populated city — a place renowned for historical landmarks and culinary culture.

After Syria’s 2011 Arab Spring protests descended into civil war, Aleppo became a key base for a number of different rebel factions opposed to the Assad regime. In September 2015, Assad's forces began a concerted effort to retake the city. By December, they had made significant advances around the city and, by February 2016, had nearly surrounded it.

Prior to the Aleppo offensive, Assad had been losing. In response, his international patrons — Russia and Iran — began increasing their support last fall. Russia sent warplanes, attack helicopters, artillery pieces, and significant numbers of military advisers. Iran sent in paramilitary operatives and battle-hardened fighters from Hezbollah, its Lebanon-based proxy.

That direct military involvement — something the Obama administration was pushed to do, but chose not to — allowed Assad's forces to make their major push toward the city.

"The operations in Aleppo Province have hinged upon heavy military support from both Russian warplanes and Iranian proxy fighters," Christopher Kozak, a research analyst at the Institute for the Study of War, wrote at the time. He continued:

Russia concentrated a significant portion of its air campaign against opposition forward positions and supply lines in Aleppo Province. Meanwhile, U.S. officials estimated in October 2015 that up to 2,000 Hezbollah, Afghan, and Iraqi Shi’a militia fighters led by Islamic Revolutionary Guard Corps (IRGC) – Quds Force commander Maj. Gen. Qassem Suleimani currently operated in Aleppo Province.

The fear, in February, was that Assad would impose an all-out siege of the city’s rebel-controlled eastern region, blocking even humanitarian assistance. In July, these fears were realized. The Assad regime, with significant support from Iran, imposed a blockade on rebel areas, cutting off supplies to some 320,000 people. The rebels briefly broke the siege in late July, but Assad’s forces reimposed it in September.

This sort of siege is illegal under international law, but Assad has been doing it for years around the country. That's because it works: Rebels who don't have food quickly lose the will or the ability to fight, and civilians in besieged areas will start cooperating with the government just to make it stop.

What we are seeing now is grim vindication of that strategy. Whether it was starvation or nonstop bombardment from Russian and Syrian warplanes, rebel defenses gradually collapsed. The Syrian Observatory for Human Rights, an anti-regime NGO, estimated that Assad’s forces had taken 90 percent of rebel holdings in eastern Aleppo by Monday. At this point, the rebels are reduced to holding a handful of blocks — and there is little hope that they will be able to fend off the regime.

Which means there is nothing standing between Assad and however many civilians remain in Aleppo.

This is a massacre, and we cannot avert our eyes

الفيديو الأخير ورسالتي الأخيرة بتمنى توصل للجمـيع وسامحونا ... 12 / 12 / 2016 #حلب_المحاصرة Posted by Joud Al Khateib on Monday, December 12, 2016

When President Obama announced the US intervention in Libya in 2011, the stated aim was to prevent a slaughter. Dictator Muammar Qaddafi’s forces were about to enter the eastern city of Benghazi, and his regime was threatening to go block to block slaughtering its residents. Obama, in a televised speech to the nation, announced that he could not let that happen.

“We knew that if we wanted — if we waited one more day, Benghazi, a city nearly the size of Charlotte, could suffer a massacre that would have reverberated across the region and stained the conscience of the world,” the president said.

American airstrikes blocked Qaddafi’s advance into Benghazi; we’ll never know how bad things would have gotten if his forces had made it into the city.

Aleppo is not so lucky.

Intervention in Syria would have been much, much more dangerous than intervention in Libya. Between potentially getting into a shooting war with Russia and the far larger contingent of jihadists in the rebel ranks, there are good reasons to believe a US bombing campaign could have made things even worse.

At the same time, one of the administration’s central arguments against intervening has been proven false. US officials, from Obama on down, had said outside military assistance wouldn't be enough to decisively impact the future course of the war. Once Russia started to bomb on Assad’s behalf, US officials said the Kremlin would get bogged down in a bloody quagmire without end. Instead, Russia — with virtually no losses of its own troops — has helped Assad retake vital ground like Aleppo while pushing his opponents closer toward near-total defeat.

Either way, there are not, and never were, easy answers to the Syrian civil war.

But that has no bearing on the suffering of people in Aleppo right now. Men, women, and children are being slaughtered by a regime that has demonstrated zero compassion for its own citizens. Iran and Russia are aiding in this mass murder. And everyone responsible appears to be getting away with it.

The world has made its choice, concluding that intervening to stop the bloodshed in Syria was too risky. Even if you think that was the right decision, you need to open your eyes to the consequences: the destruction of Aleppo, and the hundreds of thousands of civilians killed across Syria in the worst mass slaughter since the genocide in Darfur.