Now, there is a growing diplomatic consensus that Assad, the 51-year-old ophthalmologist who inherited Syria’s leadership 17 years ago from his dictator father, has almost certainly prevailed against efforts to dislodge him militarily — and that his opponents need to come to terms with his political survival as they plot a new course.

The multi-sided war, midway through a seventh brutal year, is far from over. But Assad’s consolidation of control in key parts of the country, and continued crucial aid from allies Russia and Iran, have contrived to make it virtually impossible for the rebels who once enjoyed U.S. support to drive him from power, longtime observers of the conflict say.