U.S.-backed forces ousted Islamic State from its last outpost in Syria, marking the end of a nearly five-year campaign that forced the extremist group to morph from a governing authority back into a guerrilla insurgency as swaths of territory were freed from its brutal rule.

The demise of the group’s self-proclaimed caliphate—which once covered an area the size of Portugal—comes as President Trump prepares to pull U.S. troops from Syria.

Still, Islamic State fighters in the region and the group’s followers elsewhere remain a threat, according to U.S. officials and analysts, who have warned that a drawdown of U.S. forces in Syria could allow the group to rebound.

In a monthslong campaign, the Kurdish-led Syrian Democratic Forces, backed by U.S. airstrikes, captured villages in eastern Syria’s Deir Ezzour province where Islamic State fighters had been cornered and holding out in recent months, SDF spokesman Mustafa Bali said Saturday on Twitter.

“Syrian Democratic Forces declare total elimination of so-called caliphate and 100% territorial defeat of ISIS,” he said. “On this unique day, we commemorate thousands of martyrs whose efforts made the victory possible.”


At the peak of the fight, around 10,000 Americans—8,000 troops and at least 1,600 contractors—were involved in fighting Islamic State in Syria and Iraq. At least 74 of them have been killed since 2014, according to the Defense Department.

Syrian and Iraqi troops who fought Islamic State bore far heavier casualties in the battle against the extremists, with tens of thousands dead in each country.

Tens of thousands of civilians in the two countries were also killed in the fighting, which included fierce battles in 2017 to push Islamic State out of Mosul in Iraq and from Raqqa, which was the extremists’ de facto capital in Syria.

The SDF released videos of its fighters Saturday hoisting the group’s signature yellow flag on buildings peppered with bullet holes in Baghouz, Islamic State’s last output. Pictures of the SDF’s female fighters hung on either side of the text under the group’s banners.


On Saturday afternoon, the SDF gathered for a victory ceremony at the nearby al-Omar oil field, where a march band clad in orange interspersed victory speeches with military tunes, and about 100 fighters lined up, fronted by a row of female soldiers.

From the stage, William Roebuck, the deputy special envoy to the coalition, expressed his gratitude to the SDF: “We honor their bravery and also the bravery and sacrifices of the Iraqi security forces in their fight against ISIS next door.”

Even as Islamic State has lost the towns and villages it once controlled, what are believed to be several thousand of its fighters are still active in the desert south of the provincial capital, also known as Deir Ezzour, where they are battling Iranian-backed militias. And it has reverted to insurgent tactics.

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“ISIS remains a dangerous threat in territory it does not control,” Secretary of State Mike Pompeo said in early February, citing a pair of suicide bombings in Manbij, Syria, in January, in which four Americans were among 19 people killed.


On Saturday, Mr. Pompeo, traveling in Beirut, said of Syria, “our mission there hasn’t changed. We still have work to do to make sure radical Islamic terrorism doesn’t continue to grow.”

“The end of the so-called physical caliphate is a historic military accomplishment that brought together the largest coalition in history, but the fight against Daesh and violent extremism is far from over,” said U.S. Army Lt. Gen. Paul LaCamera, commander of the U.S.-led coalition.

“Make no mistake, Daesh is preserving their force,” Gen. LaCamera added, using another name for Islamic State. “They have made calculated decisions to preserve what is left of their dwindling personnel and capabilities by taking their chances in camps for internally displaced persons and going to ground in remote areas. They are waiting for the right time to re-emerge.”

Mr. Trump warned potential targets of Islamic State propaganda, “you will be dead if you join. Think instead about having a great life,” he said in a statement Saturday.


Abu Bakr al-Baghdadi, the group’s elusive leader, who declared the establishment of the caliphate from a mosque in Mosul in 2014, is still alive and on the run. Western officials and analysts say Islamic State has been preparing for months, if not years, for life in the shadows.

“The loss of the physical caliphate is definitely a big blow to the organization,” said Amarnath Amarasingam, a senior research fellow at the U.K.-based Institute for Strategic Dialogue and expert in terrorism. “But it’s also true that most fighters expected this from the very beginning.”

Islamic State also remains a threat beyond Syria and Iraq. The group has taken responsibility for numerous attacks elsewhere in recent months, including Afghanistan, Pakistan and Egypt.

A new Islamic State faction has risen rapidly since 2016 in Nigeria, calling itself Islamic State West Africa Province, or Iswap. Well-armed and aware of the need to rally popular support, the group is viewed by local security officials as a bigger threat than the Boko Haram insurgency.

And although Islamic State’s media apparatus has taken a hit with the loss of physical territory, its propaganda in cyberspace continues to reach sympathizers world-wide, some of whom have conducted attacks in North America and Europe.

Smoke rises from a strike on Baghouz, Syria, on Friday. Photo: Maya Alleruzzo/Associated Press

Since 2014, Islamic State loyalists have conducted more than 100 attacks in dozens of countries. Among the deadliest incidents that Islamic State claimed responsibility for were the November 2015 attacks in Paris that killed 130 civilians.

As a splinter group of al Qaeda, Islamic State has its roots in the early years of the Iraq war. Around 2010, it drew on sectarian strife and discontent among Sunnis to grow. Taking advantage of weak governments in Iraq and Syria, the group fortified its ranks with prison breaks and funding from wealthy individuals in the Gulf who wanted to see Syrian President Bashar al-Assad fall.

In 2014, after sweeping through northern Iraq, Islamic State captured Mosul, one of Iraq’s largest cities, with fewer than 1,000 fighters. The group’s fearsome reputation, built on effective propaganda broadcasting its ruthlessness, helped it advance quickly as government forces often fled.

At its peak, Islamic State held contiguous territory in Syria and Iraq, where it imposed a brutal version of Islamic law and governed by collecting taxes and providing a measure of social, security and legal services.

Islamic State’s reign drove millions of Iraqis and Syrians out of their home countries, contributing to a refugee crisis in neighboring countries and Europe. The exodus has generated a political backlash in Europe and spurred the rise of politicians with anti-immigrant platforms.

Quashing the caliphate hasn’t been easy. Although Islamic State was pushed out of Iraq in late 2017, clashing interests of various forces in Syria—the U.S., Russia, Turkey, Iran and competing Syrian groups—contributed to a protracted campaign to defeat Islamic State.

Mr. Trump’s announcement in December that the U.S. would withdraw from Syria was welcomed by Russia, Turkey, Iran and the Syrian government.

But U.S. military and State Department officials have disagreed with the decision, which prompted then-Secretary of Defense Jim Mattis and Brett McGurk, the U.S. envoy to the coalition fighting Islamic State, to resign. Mr. McGurk later wrote in an op-ed for the Washington Post that Mr. Trump, by pulling troops out prematurely, was giving Islamic State new life.

The U.S. military now plans to maintain a residual force of up to 1,000 soldiers in Syria, The Wall Street Journal has reported.

During a two-day visit to Lebanon on Saturday, Secretary of State Mike Pompeo, asked by reporters whether the U.S. troop withdrawal would now begin, replied: “Our mission there hasn’t changed, we still have work to do to make sure radical Islamic terrorism doesn’t continue to grow.”

In Syria, Mr. Assad has survived the uprising against him, but he rules over a country devastated by years of conflict that extended beyond the fight against Islamic State. Chaos, instability and poor governance—all of which contributed to Islamic State’s recruiting efforts—endure.

In Iraq, the state has imprisoned thousands of militants and their families, convicting many in hasty trials and allegedly subjecting detainees to torture. Such abusive practices are ingredients for radicalization, experts say. The Iraqi government has in the past denied torture allegations.

“The situation now is actually quite similar to 2010 when it was claimed and believed that ISIS’s predecessor, the Islamic State of Iraq, was defeated,” said Peter Neumann, founder of the London-based International Center for the Study of Radicalization. “The withdrawal that came after was too hasty, and gave them the opportunity to come back.”

—Nazih Osseiran and Courtney McBride contributed to this article.

Write to Sune Engel Rasmussen at sune.rasmussen@wsj.com