It will likely come with the massacre of many surrendering fighters and innocent civilians, and take weeks to put into effect.
But now as the Syrian regime claims to have control of all of the largest city, we are dealing with yet another "unthinkable" happening.
Why was the rebel hold on eastern Aleppo so important to what people used to call the "revolution?"
It was the main urban holdout the rebels first grabbed in summer of 2012, showing they were militarily effective, and held the regime forces to a stalemate.
It was where the world witnessed the double-tap barrel bombing by the regime of children and hospitals -- by the regime -- a phrase so laden with horror, it takes a while to unpack.
It was where too you could see the slow radicalization of the rebels over time. I remember the stark difference in traveling there between 2012, when we shared cigarettes and jokes, and -- after two years of devastation -- how in 2014 our producer was interrupted mid-interview by a woman concerned that part of her hair was exposed just outside her hijab.
That's not radicalism in and of itself, but it was very distant to the secular mob we first reported on. Even then, the flags of the then-al Qaeda affiliate, the Nusra Front, were easy to spot on the street.
Moderates still lived on there, in great numbers, but like the rest of the rebels, the slow grind of bombings and civilian massacres had turned ordinary people towards a more ruthless ideology.
Where can rebels run?
This is the key issue that befalls the rebels now that they have lost this, their main bolthole of urban legitimacy. There are two places they can run, and each says something about the future of the war ahead.
First, rebel fighters can head northeast, to an enclave south of a former ISIS-held town of Jarablus, where the Turkish army has been pushing back ISIS, using Syrian moderate rebels as their ground troops.
Their progress has been slow, and rebels in these ranks say that now their fight is purely against ISIS, taking pressure off the Syrian regime. That option may appear less attractive for some fighters who have just seen their homes pounded by the Syrian and Russian advance in Aleppo.
Secondly, those that can escape could head northwest. This takes them towards the province of Idlib. Since rebel forces swept into the area in 2015, Idlib has been a complex mix of rebels.
Some are with what used to be called the Nusra Front
, which is basically al Qaeda in Syria, a group proscribed and bombed by the United States.
Others are more moderate, like Ahrar al-Sham, yet even they still face scrutiny about their radicalism.
The war has seen many groups slowly change. Take Nour al-Din al-Zinki -- a rebel group that once got US assistance, but then saw it lapse because of their radical tendencies, which culminated in some of its fighters being videoed beheading a 12-year-old
working with the regime. Al-Zinki were among the groups fighting in Aleppo's east. They may now try to flee.
This began as an uprising against tyranny, but has become an ugly war, and it has made both sides uglier as time has gone by.
Echoes of North Caucasus
This likely geographical shift of Syria's rebels to the northwest of the country will aid Russia's narrative, already in the ascendant owing to the lack of a credible Western alternative. The moderates would, in the event of this flight, be physically closer to al Qaeda, Moscow will say, lending weight to their argument that they are just fighting "terrorists."
It echoes the Russian playbook from the North Caucasus
. Initially, after the second Chechen war ended in 2000, Russia was fighting a separatist movement. Yet slowly over time Moscow eradicated the moderate leadership and killed hundreds of young Chechen men in "clean-up operations," leaving a smaller and smaller space for moderate thought.
Slowly, through the Moscow theater siege in 2002, and then the Beslan school hostage crisis
in 2004, radicals claimed the headlines. And now, more than a decade later, Russia is still fighting a small, fierce and only mildly impotent affiliate of the ISIS caliphate in its southern provinces.
In their mind, however, the plan worked. Russia removed the political space for the Chechen separatist movement, and turned all its enemies into radical terrorists it could tackle militarily with impunity.
It is, essentially, the polar policy opposite of the West's bid to seek a political middle ground and compromise. Russia seeks to dither in the political arena -- as we have seen in the turbulent but fruitless talks over a ceasefire for Aleppo in the last week -- and then pursues a flat and ferocious military solution.
Moscow's timing is also apposite. The Obama administration does not want to get involved militarily in Syria and -- in this time of transition to Donald Trump's White House -- is unlikely to act on much international outrage.
Putin seizes on US power gap
The US is about 40 days from a new President; no policy adopted now is guaranteed a long life span. Trump's choice for Secretary of State -- Rex Tillerson, a man with very close ties to Putin
-- is still some time away from his first day of work.
Russian President Vladimir Putin has seized on this gap, and hopes to present the Trump administration with a fait accompli in Syria -- one with far less US alternatives.
The question now is, where does Russia want to stop? Syrian President Bashar al-Assad has said that he hopes to regain control of all the country. That's unlikely, given the confidence of Kurdish separatists in the north in addition to the mess ISIS will leave behind.
But the Russians, along with Assad's Iranian and even Iraqi militia allies, have clearly sent a lot of hardware into the Aleppo arena. Does it all now go home or return to base? Unlikely.
The area of Idlib is home to an estimated two million people, and has al Qaeda affiliates in its midst. It is also about a 45-minute drive from Aleppo's city center. The Russians may have the firepower, will, and time to keep on going. Who is going to stop them?
In short, we are in a new phase of the civil war here. The Syrian moderate forces propped up just enough to not collapse by Gulf, American and other Western aid, are crumbling fast, if not gone from the picture.
The former rebellion is now in areas where an al Qaeda affiliate is powerful. Moscow and Damascus are finding its use of firepower against civilian areas can go unchallenged, but also now make a swift military difference.
President Obama and the coalition have done little to prevent their advance. Now, he remains silent as his days in office wind down. The US President-elect is also silent -- and has made a Kremlin regular his Secretary of State.
Russia has long been invested in Syria, and would never have relinquished its foothold in the region easily. Yet I am sure even inside the silence of the Kremlin walls you can hear a gasp of disbelief at how little the West has done to restrain them.