On the edge of Damascus, Syrian rebels have united in an effort to break the crushing siege of the suburbs known as the Ghouta. And to do so, they have rallied behind the controversial but charismatic Zahran Alloush, the Salafist head of Jeish al-Islam (the Army of Islam) and one of the most prominent rebels in Syria.

Syria’s rebels have announced alliances, mergers and charters in the past, but they have nonetheless remained largely atomized countrywide. While infighting among rebels unaffiliated with the so-called Islamic State (IS) has been exaggerated, cooperation has remained mostly ad hoc. Coordination has been hamstrung by organizational and interpersonal differences, as well as competition over funding and arms. ...