That pronouncement came at a time when Mr. Obama and some members of his inner circle thought Mr. Assad’s hold on power was shaky. But as Mr. Assad dug in his heels, Mr. Obama’s policy for Syria was marked by a chronic gap between ends and means.

The White House was slow to embrace a plan from David H. Petraeus, the former CIA director, and Hillary Clinton, Mr. Obama’s first secretary of state, to train and arm the moderate Syrian opposition. And while the Obama administration was not prepared to use airpower to protect the Syrian opposition, Russia began to send its warplanes to Syria in September 2015 to shore up the Assad government and help it reclaim lost ground.

During his final year as secretary of state, John Kerry sought to enlist Russia’s support for a diplomatic solution that would quell the fighting and provide for a political transition in which Mr. Assad would eventually leave power. But President Vladimir V. Putin of Russia decided instead to help Mr. Assad enlarge the territory he controlled. With Russian air support, the Assad government retook Aleppo in December.

The Assad government has also drawn support from Iran’s paramilitary Quds force, as well as Iranian-backed militias from Iraq, Shiite fighters from Afghanistan and Hezbollah, and the Lebanese militia.

During the presidential campaign, Mr. Trump said that he while he did not like Mr. Assad, he was glad the Syrian leader was “killing ISIS.” Since taking office, however, Mr. Trump has shied away from forging a military alliance in Syria with Russia, let alone with Mr. Assad. Instead, the administration’s single-minded focus has been to help Syrian fighters oust the Islamic State from the northern city of Raqqa, which the extremists have declared the capital of their self-styled caliphate.

Still, major questions loom, including which political authorities in Syria will control Raqqa after ISIS fighters are evicted and how the international community might establish safe zones, or what Mr. Tillerson recently called “interim zones of stability,” to stem the flow of refugees. Another important question is whether it might be possible to negotiate a broader political accommodation for Syria; despite his gains, Mr. Assad lacks the military manpower to control the entire country.