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President Trump’s decision to remove U.S. troops stationed in Syria was predicated on a seemingly simple argument: The Islamic State, the extremist group that once controlled large swatches of that country and Iraq, had been defeated.

“We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency,” the president tweeted on Dec. 19, using an acronym to refer to the group. The White House later released a video in which Trump said “we have won against ISIS.” He later appeared to modify his language — but only slightly. “ISIS is mostly gone,” he tweeted on Dec 31.

Vice President Pence reiterated that message Wednesday. “The caliphate has crumbled, and ISIS has been defeated,” he told a group of U.S. ambassadors gathered at the State Department in Washington.

Hours after U.S. service members were killed in an Islamic State-claimed attack in Syria on Jan. 16, Vice President Pence said "ISIS has been defeated." (Reuters)

Just hours earlier, a suicide bomb had killed four American military members and wounded three more in Manbij, a Syrian city patrolled by U.S. troops. The supposedly defeated Islamic State claimed responsibility.

Though the group’s involvement has not been confirmed, critics linked the attack to Trump’s decision to pull out of Syria. “Trump’s order was reckless and driven far more by domestic political concerns than it was by facts on the ground,” said Charles Lister, a senior fellow at the Middle East Institute.

So, just how “defeated” is the Islamic State? And what does defeated actually mean in these circumstances? There are no simple answers.

It’s worth remembering how powerful the Islamic State was just a few years ago. In January 2015, the group was estimated to control 35,000 square miles in Syria and Iraq — roughly the size of Maine. It had taken over major cities such as Mosul and was even bearing down on Baghdad. The next month, terrorism analyst Daveed Gartenstein-Ross estimated that the group could have close to 100,000 fighters.

The Islamic State’s self-declared caliphate was a lodestar for extremists around the world. Other organizations pledged allegiance to the group’s leader, Abu Bakr al-Baghdadi. Disaffected Westerners and other foreigners flocked to Syria and Iraq to fight, often becoming the Islamic State’s most extreme adherents. High-profile acts of terrorism took place in Western Europe and North America, roiling nations and their leaders — Trump prominent among them.

Today, there’s no denying that the Islamic State has been devastated on the battlefield. An anti-Islamic State campaign, started under the Obama administration and continued by Trump, has taken back almost all of the group’s territory. By October 2017, the once-sprawling “caliphate” was less than a quarter of the size it had been in early 2015; it soon contracted even further and lost its self-declared capital, Raqqa.

As the group’s geographical reach crumbled, so did its rhetorical power. Islamic State propaganda dwindled, and many foreign fighters were captured. In part due to the decline of the group, the number of terrorist attacks around the world has dropped for the past few years running. Trump even stopped tweeting about the Islamic State so much.

It is possible that the group, which has been known to make false claims, may not have carried out Wednesday’s attack. But the bombing would hardly be the only potential sign of its remaining power. The Islamic State is believed to have killed dozens of U.S.-backed fighters in November in Deir al-Zour, one of the group’s few remaining regional strongholds in Syria. There has also been a wave of kidnappings and assassinations in neighboring Iraq.

The Islamic State’s attempt to form a caliphate and implement its harsh form of governance has failed, but the group can still live on as an insurgency. Baghdadi is presumed to be alive, and estimates suggest there are probably tens of thousands of fighters still in Iraq and Syria. Early reports about the attack in Manbij have suggested there may have been an Islamic State sleeper cell in the city. If that’s true, there could be many more.

And in many ways, defeating an insurgent group that uses guerrilla tactics and terrorism is more difficult than beating a group that is trying to actually hold ground.

“It’s premature to use words like defeat, because we know how hard it is to truly achieve that condition,” Nicholas Rasmussen, who until December 2017 was director of the U.S. government’s National Counterterrorism Center, told The Washington Post last year. “While they are definitely degraded, ‘defeat’ suggests a condition that is irreversible or one in which ISIS no longer poses a serious threat to the U.S. Neither of those is true.”

Indeed, the Islamic State already knows how to bounce back from the jaws of defeat. Many of its core members were once members of Saddam Hussein’s army, which disintegrated during the U.S.-led invasion of the country. The group that would become the Islamic State has already recovered from near extinction after the death of its founder, Abu Musab al-Zarqawi, in 2006. This time, its leader isn’t even dead.

In a statement released later Wednesday, Pence said the United States would “never allow the remnants of ISIS to reestablish their evil and murderous caliphate — not now, not ever.”

That could yet be true: Trump has said that U.S. troops will stay in Iraq and could fight the Islamic State from there, and the United States is quietly beefing up its military presence in Qatar and Jordan. But prematurely claiming that the Islamic State has been defeated as the United States pulls out of Syria could still have real consequences.

“Trump used to say, often, he’d never telegraph his war plans or US troop movements because it could help ISIS,” NBC News’s Richard Engel tweeted. “But in Syria he’s doing just that, often.”

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