The United States’ policy options, ever bad, are more limited and less effectual than ever. The airstrikes launched last week by the United States, France and Britain sent a necessary, calibrated message to Mr. Assad that the civilized world does not countenance chemical weapons use. But their deterrent effect is likely, again, to be fleeting, not least because the United States — wary of provoking a wider conflict — has twice demonstrated that it will not use regime-threatening force to punish Syria’s use of chemical weapons.

Moreover, the United States has been deliberately ambiguous about whether our current “red line” is the use of chlorine gas alone, to which the United States and its allies have never responded with force. (American officials have said the recent attack in Douma may have involved both chlorine and a far deadlier nerve agent, sarin, stores of which Syria seems to have surreptitiously retained or reconstituted despite a 2013 agreement to eliminate them.) If the United States responds to incidents involving only chlorine, it will be striking Syria more frequently.

Most dangerously, the Syrian conflict now pits big players against one another: Israel versus Iran, the United States versus Iran and Russia, and Turkey versus American-backed Kurds. These standoffs risk escalating into sustained conflict, even if worst-case scenarios can still be avoided.