Besides, the greater threat now is not Mr. Assad but the Islamic State, especially if it continues to expand in Syria, entices more foreign fighters into its ranks and uses its territory to launch attacks on the West. A recent study by the RAND Corporation, which does research for the government, says the collapse of the Assad regime, while unlikely now, would be the “worst possible outcome” for American interests — depriving Syria of its remaining state institutions and creating more space for the Islamic State and other extremists to spread mayhem.

This was not the scenario envisioned in 2011 when Syrians staged peaceful protests against Mr. Assad’s autocratic government. President Obama and European leaders called for Mr. Assad to resign and pressured him with sanctions. The dictator, armed and aided by Russia and Iran, retaliated with his air force and barrel bombs, fueling a civil war in which some 200,000 Syrians have been killed and countless towns destroyed.

As has long been the case, the fighting in Syria raises tough questions and presents the United States with no good options. And Mr. Obama’s approach to the conflict remains the most inchoate element of his campaign against the Islamic State. While the Americans and Mr. Assad ostensibly share a common enemy, the two parties are not formally collaborating. Yet American fighter planes regularly invade Syrian airspace to bomb Islamic State targets. If the main threat is the Islamic State and the goal is to defeat it, might the West at some point be forced to work with Mr. Assad?

The administration says it is training Syrian rebels to assist the United States-led air campaign against the Islamic State, but those fighters will not be engaged for another few months and there are serious doubts about whether they can ever be effective. Figuring out Syria’s longer-term future is even more complicated. If, as American officials say, the only way to end the civil war and forge a common front against ISIS is some kind of political agreement that includes Russia and Iran, Mr. Assad’s major allies, and Turkey and Saudi Arabia, his major opponents, what must Washington do to strengthen its position and shape the outcome?

American officials see an emerging international consensus on the need for a long-term diplomatic solution between Mr. Assad and diverse rebel groups. There is also interest in United Nations-led cease-fires in local communities like Aleppo that might serve as a basis for a broader peace. As The Times has reported, the Russians are trying to bring the two sides into talks later this month, with the apparent aim of a more gradual change in Syria.