Mounting army casualties have left government supporters tired and grieving; some are resentful. There have been tensions in the southern province of Sweida, residents say, after the government tried to renege on an agreement to allow young men to serve in local defense forces and instead draft them into the army.

A Syrian who speaks regularly to security officials and leaders from Mr. Assad’s minority Alawite sect, an important component of his base, said recently that a growing number would welcome a political settlement. But with Mr. Assad’s inner circle adamantly opposed to any compromise, he said, the country will face a long insurgency.

“Nobody believes it will end in 10 years as long as he is in power,” he said, speaking on the condition of anonymity for his safety. He added that Alawites kept their discontent largely underground because most believed that their choice was between Mr. Assad and extremists bent on slaughtering them.

In Istanbul, Antakya and the nearby town of Reyhanli this month, numerous insurgents and civilian activists who oppose Mr. Assad, Nusra and Islamic State, also known as ISIS, said that most of Syria would eventually be controlled either by those extremist groups or by the government.

The fall of the army base at Wadi al-Deif, which straddles an important supply route in Idlib Province, proved the Nusra Front’s dominance, they said. Other insurgents had long besieged the base without victory. Nusra succeeded after seizing much of the province from Harakat Hazm and the Syrian Revolutionaries Front, two of several groups that until recently, American officials were calling the opposition’s new hope.

Those groups had received sophisticated American-made TOW antitank missiles, and their commanders expected to act as the ground force in the American-led campaign against ISIS. But lately they say the flow of American aid has dwindled as Washington’s strategy shifts to building a new force from scratch.