The president has some understandable reasons for this reticence. Taking on Assad is hard: He’s proven he’s immune to diplomatic pressure, and military attacks would require a major escalation of the air campaign. Action against Assad would place the United States at odds not only with Iran and Russia, which so far are not obstructing the war against the Islamic State, but also with the Iraqi government, which continues to support the Damascus regime.

The problem is that ignoring Assad is likely to lead to even worse consequences. Already, the regime and its spokesmen are exulting in the U.S. bombing raids and doing their best to portray the United States as a de facto ally, while Syrians in rebel-held areas are demonstrating against the U.S. strikes because they are seen to be weakening the resistance to Assad. Meanwhile, the regime appears to be stepping up its own bombing raids against the non­extremist opposition. A failure of the United States to respond could destroy U.S. relations not only with its only on-the-ground allies in Syria but also with the Sunni nations that have joined the campaign against the Islamic State.