On the same day ISIS claimed responsibility for a deadly attack against U.S. service members in Syria, Vice President Mike Pence declared that ISIS “has been defeated.” It is true that the U.S. and its allies have retaken virtually all of the land ISIS controlled in Iraq and Syria, but experts say the group still has tens of thousands of fighters and remains dangerous.

Pence made his remarks at the Global Chiefs of Mission Conference at the U.S. State Department.

Pence, Jan. 16: Thanks to the leadership of this commander in chief and the courage and sacrifice of our armed forces we’re now actually able to begin to hand off the fight against ISIS in Syria to our coalition partners, and we’re bringing our troops home. The caliphate has crumbled and ISIS has been defeated.

In Syria, however, 19 people were reportedly killed in a suicide bombing at a restaurant in the city of Manbij. ISIS claimed responsibility for the attack, which killed four Americans, including two U.S. service members, a Defense Department civilian and a DOD contractor, according to the Pentagon. The announcement of the U.S. deaths came shortly after Pence spoke.

After the attack, Pence released a statement offering his condolences to the “loved ones of the fallen.” He did not repeat that ISIS had been “defeated,” but he said the U.S. has “crushed the ISIS caliphate and devastated its capabilities.”

The vice president was following the lead of President Donald Trump, who on Dec. 19 posted a video on Twitter to announce that U.S. troops would be withdrawing from Syria. “We’ve beaten them and we’ve beaten them badly,” Trump said of ISIS, which is also known as the Islamic State. “We’ve taken back the land and now it’s time for our troops to come back home.”

But military experts — including those in the U.S. military — have cautioned against declaring victory over the terrorist group, even though, as Trump said, “we’ve taken back the land” once controlled by ISIS in Iraq and Syria.

According to analytics and consultancy firm IHS Markit, near its height in January 2015, the ISIS caliphate in Iraq and Syria covered about 35,000 square miles. The U.S-led coalition reported a year ago that 98 percent of that territory had been reclaimed. At the time, Brett McGurk, special presidential envoy for the global coalition to counter ISIS, said the U.S. and its allies had made “a lot of progress,” but warned that “ISIS will be around for a while, so … we have a long way to go.”

In a September 2018 report, the nonpartisan Congressional Research Service — citing U.S. military estimates — said ISIS had “approximately 30,000 current and former IS personnel” in areas of Syria and Iraq. The United Nations made “similar estimates” of ISIS presence in the region, the report said.

The CRS report quoted a Defense Department spokesman, Cmdr. Sean Robertson, as saying ISIS is probably as dangerous as al-Qaeda in Iraq was at its peak in the mid-2000s.

CRS report, Sept. 25, 2018: Defense Department officials assess that the Islamic State “is well-positioned to rebuild and work on enabling its physical caliphate to re-emerge,” and “probably is still more capable than Al Qaeda in Iraq at its peak in 2006-2007, when the group had declared an Islamic state and operated under the name Islamic State of Iraq. …”

Those partial quotes are from Robertson’s interview with Voice of America in August for a story that carried the headline, “Islamic State ‘Well-Positioned’ to Rebuild Caliphate.”

On Dec. 11, 2018, just eight days before Trump announced a troop withdrawal from Syria, McGurk said at a press briefing, “Nobody is declaring a mission accomplished. Defeating a physical caliphate is one phase of a much longer-term campaign.”

In a Dec. 15 interview, four days before Trump’s announcement, McGurk told CNBC that there was still more work to do in the months ahead to keep control of recaptured land.

“We’re on track now over the coming months to defeat what used to be the physical space that ISIS controlled,” McGurk told CNBC’s Hadley Gamble. “That will not be the end of ISIS.”

After Trump’s decision to withdraw, Defense Secretary James Mattis and McGurk both resigned in protest.

Daniel Byman, a senior fellow in the Brookings Institutions’ Center for Middle East Policy and the senior associate dean for undergraduate affairs at Georgetown University’s Walsh School of Foreign Service, wrote in December, after Trump’s withdrawal announcement, that the Islamic State is “far from ‘defeated,'” as the president had claimed.

“The U.S.-led military campaign has greatly weakened the group, and Trump should rightly be proud of that success,” Byman wrote in an article for Brookings’ website. But the group could reemerge.

“The Islamic State still controls small pockets of territory in Syria. More important, it has a large underground presence, and it is waiting for outside pressure to ease to reassert itself,” Byman said. “The group commits dozens of attacks each month as well as a massive assassination campaign to prevent any local governance from taking hold, and it still has thousands of fighters under arms.”

A month earlier, Byman wrote that ISIS, as other terrorist groups before it, has spread its reach beyond Iraq and Syria to other countries.

“[T]he Islamic State and other groups like al-Qaida have a presence in countries like Mali, Pakistan, Somalia, and other parts of the Muslim world,” he wrote. “Though outflows of foreign fighters from Syria have not been huge, small numbers have made their way to Libya, Afghanistan, and other fields of jihad.”

Similarly, Michael P. Dempsey, the national intelligence fellow at the Council on Foreign Relations and the former acting director of National Intelligence, wrote a year ago that the group’s “tactics are evolving.”

“With the end of the physical caliphate, ISIS’ tactics are evolving. It is more and more likely to avoid major battlefield engagements and instead resort to terrorist attacks in the Middle East, other conflict zones, and the West,” Dempsey wrote in Foreign Affairs, a publication of the Council on Foreign Relations.

Dempsey continues: “U.S. policy needs to change quickly to meet the evolving threat, both in terms of its operations in the region and of its counterterrorism priorities at home.”

“In order to remain militarily relevant, ISIS increasingly prefers to conduct isolated suicide attacks and hit-and-run operations,” Dempsey wrote. “In early January, the group’s official media wing published a list celebrating nearly 800 such attacks in 2017, including ones against the Iraqi military (nearly 500), Kurdish forces in Syria (136), and the Assad regime and its allies (120), as well as a few dozen against moderate opposition groups in Syria.”

As far back as December 2017, Benjamin Bahney and Patrick B. Johnston wrote in Foreign Affairs that ISIS was “quietly preparing” to resurrect and reinvent itself.

“ISIS could resurrect its caliphate where it was born, in Iraq and Syria. It has been planning for such a resurrection since at least 2016, and quietly preparing since well before losing Raqqa in October,” they wrote. “Most ominously, ISIS has a tried-and-true playbook for bringing itself back from near death. Just a few years ago, it managed to resurrect itself after apparent defeat. And the history of that resurrection should serve as a warning of what may be coming now.”

It’s unclear how and when the Trump administration will withdraw troops from Syria. John Bolton, the president’s national security adviser, has said the U.S. will impose conditions for withdrawal.

“There are objectives that we want to accomplish that condition the withdrawal,” Bolton told reporters in Jerusalem on Jan. 6. Bolton’s conditions included “defeating what’s left of ISIS in Syria and protecting Kurdish militias who have fought alongside U.S. troops against the extremist group,” according to the Associated Press.