In April, President Trump vowed to bring American troops home from Syria. “I want to get out,” he said during a press conference. The United States had spent trillions of dollars in the Middle East over the past seventeen years, he complained. “We get nothing—nothing out of it.” He called it “a horrible thing.” The United States had been “very successful against ISIS,” he said, “but sometimes it’s time to come back home.” By then, ninety-five per cent of the ISIS pseudo-caliphate in Syria and Iraq—once the size of Indiana—had been liberated. U.S. officials claimed that tens of thousands of ISIS fighters had been killed; a residual force, no more than three thousand strong, was isolated in two small pockets of Syria near the Iraqi border. The goal was to get the two thousand U.S. troops, pivotal in providing strategy and intelligence to the Syrian rebels fighting ISIS, out in the early fall.

No longer. ISIS is now making a comeback. The frequency of the group’s attacks is up, and so, apparently, are its numbers. It excels, once again, at crafting small explosive devices, and weaponizing drones. And its sophisticated media outreach is recovering, according to a new U.N. report. The elusive ISIS emir, Abu Bakr al-Baghdadi, whom Russia claimed to have killed in a May, 2017, air strike, reëmerged this month with an hour-long broadcast, his first in a year. He acknowledged that his followers had been tested with “fear and hunger.”

The United States had boasted of its “so-called victory in expelling the state from the cities and countryside in Iraq and Syria,” Baghdadi, who was held by U.S. forces in Iraq for several months in 2004, said. He urged a different metric. “The land of Allah is wide and the tides of war change,” he said. “For the believer mujahideen, the scale of victory or defeat is not tied to a city or town being stolen or subject to those who have aerial superiority, or intercontinental missiles or smart bombs.” He referred to the revival of an earlier version of ISIS after it was decimated by U.S. troops in Iraq during the surge of 2007. At the time, the jihadi group was down to only a thousand fighters. ISIS subsequently mobilized more than sixty thousand fighters from more than a hundred countries to its cause. Baghdadi vowed that those who “patiently persevere” would again have “glad tidings.”

ISIS may already have numbers sufficient to rebuild. Two stunning reports this month—by the United Nations and Trump’s own Defense Department—both contradict earlier U.S. claims that most ISIS fighters had been eliminated. The Sunni jihadi movement still has between twenty thousand and thirty thousand members on the loose in Iraq and Syria, including “thousands of active foreign terrorist fighters,” the U.N. said, despite the fall of its nominal capital, Raqqa, last October. The Pentagon report is more alarming: ISIS has fourteen thousand fighters—not just members—in Syria, with up to seventeen thousand in Iraq. More important, ISIS has successfully morphed from a proto-state into a “covert global network, with a weakened yet enduring core” in Iraq and Syria, with regional affiliates in the Middle East, Africa, and Asia, the U.N. reports. It can “easily” obtain arms in areas with weak governance; it is now a threat to U.N. member states on five continents.

So the Trump Administration has reversed course; it is now keeping U.S. troops in Syria indefinitely. “We’re remaining in Syria,” Brett McGurk, the Special Presidential Envoy for the Global Coalition to Counter ISIS, told reporters on August 17th. “The focus is the enduring defeat of ISIS. We still have not launched the final phase to defeat the physical caliphate. That is actually being prepared now, and that will come at a time of our choosing, but it is coming.” (ISIS fighters are holed up in the Middle Euphrates Valley, including around the small city of Hajin.) U.S. troops will also need to train local forces to hold the ground so ISIS cannot return, McGurk said. “So this mission is ongoing.”

The third and final phase of the U.S.-orchestrated campaign against ISIS is expected to take several months, a U.S. official told me. The goal is for the Syrian Democratic Forces, backed by U.S. airpower, to liberate about four hundred square miles in northeast Syria along the Iraqi border. (The United States has already launched almost thirty thousand air strikes on ISIS targets in Syria and Iraq, at a cost of more than thirteen million dollars a day.) But U.S. officials acknowledge that that alone will not eliminate ISIS. It is also active in other areas south of the Euphrates River and near the city of Palmyra, which is under the control of President Bashar al-Assad’s regime and its allies—where the U.S. does not have a presence.

The U.S. intelligence community is deeply divided, however, over the scope of the ISIS threat—and even the numbers put out in the Pentagon report. One issue is how to count ISIS: by fighters on the battlefront, or anyone who has worked in some capacity for the Islamic State’s caliphate or is sympathetic to it? Other agencies, including the C.I.A., have offered estimates that conflict with the Defense Intelligence Agency (D.I.A.), a U.S. official told me. “There’s a very big discrepancy.”

Even top military officials have cast doubt on the Pentagon report, which was assembled by the Inspector General based on D.I.A. reporting. “I don’t have high confidence in those particular numbers,” General Joseph Dunford, the chairman of the Joint Chiefs of Staff, said this week. “Over the last two and a half years, ISIS has lost about ninety-eight per cent of the ground that they’ve held. They’ve lost significant access to resources, and the flow of foreign fighters has been significantly reduced. Those are all quantifiable, and we know that.”

The numbers game echoes the problem the U.S. faced during the Vietnam War—namely, what constitutes an effective strategy, a military victory, or even a fighter. According to an article by the Soufan Center, which was founded by Ali Soufan, a former F.B.I. terrorism specialist, “From at least 2014, estimates of the group’s strength have varied greatly, with the coalition against the Islamic State having somehow reportedly killed 100% of the group’s estimated members several times.” The center suggests “the only clear conclusion from the lack of clarity is that IS was and remains likely much larger than many had reported” and remains “among the most powerful terrorist groups in history, with no shortage of weapons or willing recruits.”

The danger of using either numbers or territory as a barometer is evident in Iraq. In December, Iraqi Prime Minister Haider al-Abadi declared victory over ISIS. “Our forces fully control the Iraqi-Syrian border, and thus we can announce the end of the war against Daesh,” al-Abadi said, using the Arabic acronym for the Islamic State. “Our battle was with the enemy that wanted to kill our civilization, but we have won with our unity and determination.”