The disillusionment of Abu Khattab — who asked that his full name be withheld because he still fears retribution from jihadists — helps illustrate the great challenge now facing Saudi Arabia’s rulers: how to fight an increasingly bloody and chaotic proxy war in Syria using zealot militia fighters over whom they have almost no control.

The Saudis fear the rise of Al Qaeda’s affiliates in Syria, and they have not forgotten what happened when Saudi militants who had fought in Afghanistan returned home to wage a domestic insurgency a decade ago. They officially prohibit their citizens from going to Syria for jihad, but the ban is not enforced; at least a thousand have gone, according to Interior Ministry officials, including some from prominent families.

But the Saudis are also bent on ousting Syria’s president, Bashar al-Assad, and his patron, Iran, which they see as a mortal enemy. Their only real means of fighting them is through military and financial support to the Syrian rebels. And the most effective of those rebels are Islamists whose creed — rooted in the puritanical strain of Islam practiced in Saudi Arabia — is often scarcely separable from Al Qaeda’s.