Aleppo has fallen.

In December, after an extended siege and a month of intense fighting, Bashar al-Assad’s forces seized control of the rebel-held eastern half of the city. A withdrawal agreement allowed thousands of rebels and civilians to evacuate — but not before Assad’s forces massacred and tortured an unknown number of people.

The Assad victory was a humanitarian disaster and a devastating blow to the embattled anti-government rebels — and something that was in the works for over a year. The above video explains why.

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 populous city — a place renowned for its 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 the summer of 2015, the rebel presence there was solid. In fact, Assad was widely understood to be losing ground around the country — and possibly his grip on power.

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.

This support turned the tide. In September 2015, Assad's forces were strong enough to launch a concerted effort to seize Aleppo and the surrounding environs. By December, they had made significant advances around the city and, by February 2016, had nearly surrounded it.

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. Assad also launched a campaign to systematically destroy the medical facilities in rebel-held parts of the city, killing or wounding many of its remaining doctors and nurses.

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 often start cooperating with the government just to make it stop. Western powers, meanwhile, have shown no inclination to try to stop it.

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 December 12. The rest of the city fell shortly thereafter — and now the consequences are on display, for all the world to see.

This is a catastrophe

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. Men, women, and children are being slaughtered and forced from their homes by a regime that has demonstrated zero compassion for its own citizens. Iran and Russia are aiding in this atrocity. 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.