By repudiating Mr. Assad without any nuance, the administration complicated its ability to negotiate with minority Kurds, Christians and Druse, who are suspicious of Mr. Assad but even more fearful about the uncertainty that would accompany a takeover by Sunni-led rebels — many of whom espouse a strict interpretation of Islam, and some of whom have openly affiliated with Al Qaeda. Their participation is essential to any future Syria.

Our black-and-white stance on Mr. Assad was not carefully arrived at. It has boxed us in and left us with the unenviable task of refereeing among opposition groups whose capacity for disagreement seems unlimited.

None of this in any way exonerates Mr. Assad, who should face prosecution for war crimes. But the armchair hawks who are boldly calling for bombing the parts of Syria controlled by Mr. Assad, or directly arming the rebels, ought to pause and reflect on who would rule Syria if he were toppled tomorrow…

Unspeakable atrocities are occurring in Syria as both sides — the rebels, backed by Saudi Arabia, Qatar and Turkey, and the Assad regime, supported by Iran and Russia — continue to fight a war with no end in sight. If both sides come to believe that neither is likely to achieve total victory, and that the only realistic outcome is a negotiated settlement, there is a chance they will settle. No one ever wants to be the last to die in a civil war.