Those trying to negotiate from the outside an end to the fighting in Syria behave as though some diplomatic elite or mix of power brokers could restore stability if only Bashar al-Assad would leave and the U.S. and Russia could agree on how to approach negotiations. But Syria is being ravaged by four broad sets of fighters that have little reason to cooperate with any U.N.-led negotiating effort—or each other.

The problem is not simply Mr. Assad or Islamic State. ISIS occupies parts of Syria and Iraq and continues to systematically purge any religious and ideological dissent. Meanwhile, the governments in Damascus and Baghdad have shown no ability to gain support from a major portion of the Sunnis in ISIS-controlled areas.