Western-backed fighters in southwestern Syria, the one part of the country where they are still strong, have spoken out against al Qaeda, a move could lead to fighting among of President Bashar al-Assad.

The most powerful opponents of Assad in Syria’s civil war are Sunni Muslim jihadists from two groups: the Islamic State, also known as ISIL or ISIS, and al Qaeda’s Syria branch, the Nusra Front. Western and Arab countries which oppose both Assad and the jihadists aim to support what Washington calls “moderate” rebels.

Although such Western-backed fighters control comparatively little territory, an alliance known as the Southern Front has an important foothold near the borders with Jordan andIsrael. It has seized a border crossing and a government-held town in recent weeks after weathering a government offensive. […]