The last 12 months have seen the Syrian Arab Army (aided by Russia, Iran and various militias) make spectacular territorial gains against the Islamic State and Sunni rebel groups—and do so without disturbing the tenuous power balance that characterizes the present conflict. Russia, Iran, the United States and Turkey all have troops on Syrian soil, and Israeli jets are no stranger to the airspace (MEES, 15 June).

President Bashar al-Assad is determined to return all of Syria to...