But as Assad’s army weakens, rebels and analysts warn, it is also becoming more extreme in its fight for survival. And two of the grimmest scenarios observers have long feared—that Islamic extremism could come to dominate the rebel fight, and that Assad could decide to attack with chemical weapons—now look more likely than ever to take hold. …

But the army has been on the defensive for months against an opposition that—though disjointed—has managed to carve out major gains in Syria’s countryside and cities. In recent weeks, the rebels have enjoyed a surge in momentum, overrunning key military bases around the country and even using anti-aircraft missiles to shoot army planes from the skies. At the same time, rebel forces have pushed ever deeper into Damascus. Not long ago, the capital was considered so secure as to beyond the opposition’s reach. Now, international airlines are cancelling flights to the city’s airport for fear of the intensifying conflict. The United Nations has pulled staff from the capital, and diplomats have fled.

In the midst of such high-profile successes, the rebels might be expected to boast about gaining the upper hand, as they’ve often done in the past. But for many, the recent progress in Damascus has been accompanied by a lingering sense of dread. Rebel fighters in the city suspect that they are nearing the government’s inner sanctum and that the fight to defend it will be vicious. Mattar Ismail, the spokesman for the formidable Ahfad al-Rasul rebel brigade in Damascus, says that the coming days will see a new surge of violence unleashed on the capital. “There will be a sudden increase in the regime army’s use of force,” he says. “We will see more destruction and more victims.”