IRBID, Jordan—Before Russia started its bombing campaign in Syria in September, Syria’s moderate opposition bet a military offensive in the south of the country could change the course of the war and force President Bashar al-Assad to the negotiating table.

That summer offensive collapsed, bolstering Mr. Assad’s regime and depleting the ranks of mainstream rebel forces already struggling to stay relevant in Syria’s future. Mr. Assad and his Iranian and Russian patrons used the defeat to again portray the war as a fight against...