The initial plan was dubious. The new one is hallucinatory, and it is being rolled out as the war enters a more perilous phase now that Russia has significantly stepped up its military support of Mr. Assad’s forces.

There is no reason to believe that the United States will suddenly be successful in finding rebel groups that share its narrow goal of weakening the Islamic State, also known as ISIS or ISIL, but not joining the effort to topple Mr. Assad. Washington’s experience in Syria and other recent wars shows that proxy fighters are usually fickle and that weapons thrust into a war with no real oversight often end up having disastrous effects.

Moscow recently began bombing rebel-held areas in strikes that appeared designed to cripple the opposition groups that pose the gravest threat to the Syrian government. That approach will almost certainly prove to be self-defeating even if the Assad regime, after gassing and slaughtering thousands of Syrians, were to stay in power in the short run. It will fuel the appeal of Sunni extremist groups that emerged from Syria’s popular rebellion and have metastasized into a global menace.

As things stand now, the Obama administration can aspire to little more than maintaining a tenuous stalemate in the fight against the Islamic State as it waits for other fronts of the conflict to shift. Meanwhile, with the United States and Russia carrying on divergent air bombing campaigns, the risk of an inadvertent confrontation between the two militaries grows more likely by the day.

Getting out of the quagmire in Syria may appear harder than it has ever been. But the only viable solution remains a diplomatic breakthrough that leads to a transfer of power in Damascus and paves the way for a unified campaign against the Islamic State.