BEIRUT, Lebanon — Make life intolerable and death likely. Open an escape route, or offer a deal to those who leave or surrender. Let people trickle out. Kill whoever stays. Repeat until a deserted cityscape is yours.

It is a strategy that both the Syrian government and its Russian allies have long embraced to subdue Syrian rebels, largely by crushing the civilian populations that support them.

But in the past few days, as hopes for a revived cease-fire have disintegrated at the United Nations, the Syrians and Russians seem to be mobilizing to apply this kill-all-who-resist strategy to the most ambitious target yet: the rebel-held sections of the divided metropolis of Aleppo.

The killing and destruction in Syria, of course, has stupefied much of the world over the past five years. But it could pale in comparison with a military assault to retake all of Aleppo, once Syria’s largest city and still home to about two million people, roughly 250,000 of them in rebel-held territory.