If the insurgents cement their hold on Idlib — as they appeared to be doing in videos that showed them in control of government buildings in the city’s center — it would be only the second time during the war that the government has entirely lost control of a provincial capital, after the loss of Raqqa two years ago…

Given the dire experience of other cities during the war, there is a formidable list of ways things could go wrong.

The government could unleash a new campaign of aerial bombing, the tactic it has used in insurgent-held areas of Aleppo and in the Damascus suburbs. Insurgents could take revenge on pro-government residents. Or Nusra, seeking to compete with the Islamic State, could attempt to declare an emirate, its answer to ISIS’s self-declared caliphate.

Nusra and the other groups that participated in the takeover oppose ISIS, but Nusra and others have their own records of extremism and brutality. The operation, which fighters said was planned for months, was directed, they said, by a new organization formed for the battle, called Jaish al-Fatah. Under it were Nusra, Ahrar al-Sham, an ultraconservative group, and other Islamist groups. In previous joint operations Nusra, which includes foreign fighters and Syrians, has influence greater than its numbers because it is willing to use suicide bombers.