The United States is increasingly ill-positioned to prevent a resurgence and expansion of the Islamic State despite the welcome tactical success and symbolic importance of the U.S. raid that eliminated the militant group’s top leader, according to a wide range of regional experts and former defense and intelligence officials.

Senior administration officials hailed the operation that killed Abu Bakr al-Baghdadi, following the declared destruction of the Islamic State’s caliphate in Syria and Iraq, as evidence that the United States remains determined to eliminate the militant group.

“The important thing is to celebrate the fact that the head, the founder of ISIS is dead. We got him. It signifies our commitment to the enduring defeat of ISIS,” said Defense Secretary Mark T. Esper, using another name for the Islamic State.

In President Trump's Oct. 27 remarks, he described an operation involving "helicopters...ships and planes," and said he watched it from the Situation Room. (Sarah Parnass/The Washington Post)

But the raid came amid rising concern that the diminishing U.S. military and civilian footprint, along with cuts in funding for stabilization and reconstruction, undermines that commitment in a part of the world where U.S. leadership is crucial both to American and global security.

President Trump has repeatedly said that other regional actors should take over the expense and physical presence of the United States in Syria. Last year, Trump cut hundreds of millions of dollars in stabilization aid to areas where U.S. air power and American-advised and assisted Syrian Kurdish ground forces drove the Islamic State out of northern Syria. With the announced departure of U.S. troops in recent weeks, the administration has appeared to encourage the expansion of Turkey and Syrian government forces, along with their Russian allies, into the region.

None of these actors is seen as having the ability or the will to command the international coalition that brought down the caliphate or to lead and equip the Kurdish-dominated Syrian Democratic Forces.

President Trump thanked Russia and Iraq for their involvement in the death of Islamic State leader Abu Bakr al-Baghdadi from the White House on Oct. 27. (The Washington Post)

Turkey’s main interest is in replacing the Kurds with Syrian refugees, and Syrian President Bashar al-Assad is unlikely to address the grievances of his citizens that helped bring the rise of the Islamic State in the first place.

Seth G. Jones , a former adviser to the U.S. Special Operations Command, said the United States “has got to make a choice”: supporting, “directly or indirectly,” the Syrian government in maintaining or taking back territory in the face of an Islamic State resurgence or supporting local actors in doing so.

“The problem is the United States has decided for the moment to do neither,” said Jones, now of the Center for Strategic and International Studies. “Walking away does not solve this problem.”

There are parts of the world where the United States can play a secondary role in supporting the efforts of other countries to fight terrorist groups, as it has done in West Africa, where France has taken the lead, several experts said.

“At one level, Trump is right in that I would love it if there was another entity willing and capable” of taking on the task of combating the Islamic State in its core area of Syria and Iraq, said William F. Wechsler of the Atlantic Council, a former deputy assistant secretary of defense for special operations and combating terrorism.

But “there is nobody else in the area that we’re talking about right now,” Wechsler said. If there were, he said, “ISIS would not have emerged in the first place. We are left with this same policy conundrum. As much as we want to hope” that Islamic State resurgence “is not the case, reality and experience tells us otherwise. We’re forced to confront these issues sooner rather than later.”

Russia, which apparently agreed to allow U.S. aircraft conducting the Baghdadi raid to fly through airspace it controls in northwestern Idlib province, “is not an ally of the United States. The president doesn’t believe that. I don’t believe that,” White House national security adviser Robert C. O’Brien said Sunday on NBC’s Meet the Press. But, he added, “when our interests overlap with Russia, there’s no reason why we shouldn’t work with them.”

Russia, however, has shown little interest in a sustained counterterrorism campaign in Syria, even if U.S. policy were to allow Russian dominance in the region.

In a news conference Sunday, SDF spokesman Redur Zelil said operations against the Islamic State would continue and “we will eliminate their sleeper cells across the region.” But the shape of that effort remained unclear.

In addition to monitoring the militants’ remnants across the northeast, the area’s mostly Kurdish administration is responsible for thousands of Islamic State prisoners, many of them foreigners.

In a visit to one of those facilities Sunday, prison guards described for a reporter how security had deteriorated since the Turkish assault. “Half of our guards were transferred to the front line,” said one, speaking on the condition of anonymity because he was not authorized to talk with the news media.

Reports of Baghdadi’s death had yet to reach the inmates, and prisoners inside one cell asked reporters repeatedly for news from the outside. Guards have disconnected their television sets in recent weeks and family visits — where they have been allowed — have been suspended, prison authorities said.

“We don’t want to give them anything that will make them riot,” the guard said.

But intelligence and counterterrorism experts said the U.S. pullout from numerous areas of northern Syria — beyond the encirclement of the oil area held by the SDF far from Kurdish population areas — would make it more difficult to conduct operations like the Baghdadi mission, which depend heavily on human intelligence networks in the country.

The Kurds, who intelligence officials said provided crucial information in locating Baghdadi, have long been a key intelligence-sharing partner, gathering information from their own networks of human sources, current and former intelligence officials said. Trump acknowledged their contributions in his own statement.

But “it takes months, if not years, to perfect the U.S. government’s ability to manhunt in specific, semi-denied areas of operations” like Syria, said Marc Polymeropoulos, a former CIA official with extensive counterterrorism experience overseas who retired this year.

“Fusing reports from our allies, human and signals intelligence and reconnaissance takes time, serious analytical work and patience,” he said. “We will lose this critical edge with a withdrawal of forces.

“It’s a great day,” Polymeropoulos said of the Baghdadi killing, “but a clear reminder of why we actually need to remain in Syria, even in small numbers, as long as ISIS remains a threat.”

Daniel Hoffman, a former CIA officer who worked on counterterrorism operations in the Middle East, agreed. “You’ve got to go out and meet sources or have liaison partners like the Kurds,” Hoffman said. “But if you’re not there physically, then it’s really hard to conduct the human intelligence-gathering that . . . is the backbone for conducting these kinds of raids.”

Experts differ on how quickly the Islamic State may be able to reformulate its leadership after Baghdadi’s death and reactivate partially dormant cells into a fully resurgent terrorist group.

“When you think about the impact it will have on ISIS going forward, this is more like the close of a chapter, but by no means the end of the story,” said Nick Rasmussen, the former head of the National Counterterrorism Center during the last two years of President Barack Obama’s tenure and the first year of the Trump administration.

“In terms of the immediate impact on the lethality of the group . . . or the degree to which ISIS as an organization or a global enterprise threatens the United States, that doesn’t change in the immediate aftermath of something like this,” he said.

An estimated tens of thousands of Islamic State fighters remain in Iraq and Syria, conducting sporadic attacks in both countries. Islamic State cells of varying activity now exist in 14 “provinces” or countries, according to the Institute for the Study of War, which described a presence that “provides footholds from which to further metastasize, launch attacks, and gain resources to fund its resurgence in Iraq and Syria.”

One key question is whether Baghdadi’s demise in Idlib province, where a small number of Islamic State fighters are located, will lead to an alliance with other extremist organizations there that have a much higher profile.

The province is largely controlled by Hayat Tahrir al-Sham (HTS), a militant group that has reinvented itself several times during the civil war. After HTS broke with al-Qaeda last year, more extreme militants left the group to form a smaller, hard-line faction, Hurras al-Din, which is the new al-Qaeda affiliate.

“In the face of the kind of pressure and the mutual threats that they’ve experienced, I wouldn’t be surprised to see more cooperation, even if it doesn’t mean a public alliance” among the extremists, said Tricia Bacon, a former State Department counterterrorism official now at American University.

Correction: An earlier version of this story misstated the last name of a former State Department counterterrorism official. Her name is Tricia Bacon, not Tricia Burton.

Loveluck reported from northern Syria. Ellen Nakashima in Washington, Sarah Dadouch in Beirut and Kareem Fahim in Istanbul contributed to this report.