The recent diplomatic activity on Syria is intriguing. Russia, Mr. Assad’s most powerful supporter, has forged new ties with Saudi Arabia, his bitter foe and a major funder of Syrian rebel groups, and brokered a meeting between Syrian and Saudi intelligence officials. Earlier this month, the Russian and Saudi foreign ministers met in Moscow while Mohammad Javad Zarif, the foreign minister of Iran, Mr. Assad’s other main ally, was in Damascus to see Mr. Assad. A week before that, Secretary of State John Kerry and his Russian and Saudi counterparts held their first three-way meeting on Syria.

Image The aftermath of airstrikes by Syrian government forces in Douma, Syria, on Aug. 16. Credit... Sameer Al-Doumy/Agence France-Presse — Getty Images

There are compelling reasons to stave off a further collapse of the Syrian state: Mr. Assad, struggling on the battlefield and in recruiting troops for his army, is growing weaker. The Americans, the Russians and the Saudis, among others, fear that ISIS, which already controls a large portion of Syria and is stronger than other militant groups there, will take over if the regime falls.

Meanwhile, Mr. Obama’s plans to equip and train a Syrian rebel ground force that could capitalize on American airstrikes against ISIS by securing territory has largely failed. Insurgent groups that were seen as potential partners in the anti-ISIS fight remain unorganized and are often at war with one another.

Even so, despite the importance of the moment, the diplomatic maneuvering seems ad hoc, with the parties testing the waters rather than developing a fully conceived plan for moving forward. The only concrete progress has been a modest nonbinding statement adopted by the United Nations Security Council last week, urging the Syrian government and its opponents to discuss a “political transition” along the lines of what diplomats agreed to in Geneva in 2012.

The obstacles to an agreement on such a transition are formidable, not least because of the absence of any consensus on how long it might last and when, if ever, Mr. Assad might step down. The United States and others, including Saudi Arabia and Turkey, have insisted that he go, while Russia and Iran have stuck by him. American officials believe Russia might be willing to cut ties with Mr. Assad as part of a plan that stabilized Syria without him and prevented an ISIS takeover, but so far none of the plans put forward have won acceptance.