Ryan C. Crocker, a veteran diplomat in the Middle East, including in Lebanon, Syria, Kuwait and Iraq, where he served as an American ambassador, said he believed that the fighting in Syria would go on for years because once the Assad government had taken the cities, the insurgents would hide in the countryside.

“The Lebanon civil war is a comparison worth looking at,” he said. “It was long, hot and mean, and it took 15 years to end and it only ended because the Syrians moved into Lebanon and stopped it.”

He added, “With Syria, we’re just five years into it, and there’s no Syria to come in and end it.”…

But the darker side is what kind of country would be left. “So Assad stays there and the Russians and Iranians prevail, but they govern over a half-dead corpse, and Syria is just this gaping wound that stretches as far as the eye can see,” Mr. Ford said.

Mr. Assad would also be beholden to his two sponsors, Russia and Iran, reviled by many of his own citizens in the Sunni-majority country and rejected by some of the main Sunni powers in the Middle East. That could mean he would face efforts from Iran to solidify its regional reach by expanding Shiite influence in Syria and demanding a role in conquered areas such as Aleppo, perhaps even assigning Iranian-backed Shiite militias there, some experts said.