Defense Secretary Jim Mattis, in testimony last week before Congress, said the administration did not yet have “a fully fleshed out” strategy for maintaining stability in Syria and Iraq after the Islamic State is defeated.

Mr. Mattis said he was consulting with Secretary of State Rex Tillerson on a larger strategy that includes both diplomatic and military components. “His diplomats are literally serving alongside us in Syria right now with our officers who are in that fight,” Mr. Mattis told the House Armed Services Committee. “So I am confident it’s being put together. It’s not complete yet.”

A State Department officer has rotated through Syria over the past 18 months, reporting on the political situation in the accompanying United States Special Operations forces who are advising American-backed Syrian Arab and Kurdish fighters combating the Islamic State. As those militias have reclaimed towns and villages in eastern Syria in recent months, and are now poised to recapture Raqqa, the Islamic State’s self-declared capital, in the coming months, a sense of urgency has grown about addressing post-conflict priorities, including ensuring governance and providing aid to more than 400,000 civilians in the Raqqa province that the United Nations High Commissioner for Refugees has cited as in need.

The looming problems in Syria are daunting. Unlike in Iraq, there is no functioning government or security force in the predominantly Arab areas that the American-backed fighters are about to take back from the Islamic State.

“In Iraq, you have got a police force and court system, which are not perfect but at least exist,” Mr. Dobbins said. “In Syria, there is no comparable authority to whom you can hand off these problems.”

Adding to the challenge, neither the United States nor other nations are eager to commit significant funds to reconstructing a Syria that is run by President Bashar al-Assad. Nor is the United States interested in remaining as an occupying power as it did for years in Iraq.

Another consideration, said Linda Robinson, a senior international policy analyst at the RAND Corporation, is that a major civilian American presence to advise on the governing of newly liberated area might provoke a backlash.