It may take weeks or months, but Aleppo is likely to fall to Syrian government forces backed by Russian air power and the most lethal bombardment in nearly six years of war.
Capturing the strategically important city, an economic and trading center which is key to controlling Syria's northwest, would be an important military triumph for President Bashar al-Assad and his Russian and Iranian allies.
It would be a crippling setback for the Western-backed Syrian rebels who, without quick reinforcements from their foreign backers, look set to be bombed out of their stronghold.
But the fall of Aleppo will not mean an end to the war, military and political analysts say.
Instead it is likely to give way to a long-term Sunni guerrilla insurgency in which the remaining moderate rebel groups, backed by the West and the West's regional allies, are driven into the arms of militant jihadis.