The alleged defeat of the group by anti-ISIS coalition forces was Trump’s main argument to justify his decision to withdraw U.S. military personnel from Syria, despite objections by his own advisers.

Less than a month later, on Wednesday, four Americans were killed in an attack in the Syrian city of Manbij, inevitably raising new doubts over any assessment that the Islamic State is beaten. The Islamic State could still succeed in reinventing itself, my colleague Adam Taylor writes, noting that “defeating an insurgent group that uses guerrilla tactics and terrorism is more difficult than beating a group that is trying to actually hold ground.”

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A look back in history shows that it certainly would not be the first group to morph into something different while maintaining its deadly mission.

The most recent reminder of that came this week, when Somalia’s al-Shabab terrorist group claimed responsibility for an attack on a Nairobi hotel that left at least 21 people dead. Such claims can be misleading, but the group was also behind the somewhat similar 2013 attack on Kenya’s Westgate shopping mall, in which 67 victims died. It was one of the deadliest post-9/11 attacks

Only one year before the Westgate attack, in 2012, African Union forces had announced victory over the group in Somalia’s capital, Mogadishu, predicting an imminent defeat of the militants in all remaining parts of the country.

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To understand why the Islamic State may not be as defeated as Trump claims it is, it’s worth exploring the group’s own history. It did not simply appear out of nowhere in 2013, but instead emerged out of its forerunner, known as al-Qaeda in Iraq, a group founded by Abu Musab al-Zarqawi around 2004 following the U.S. invasion of Iraq. After Zarqawi was killed in a U.S. airstrike in 2006, the group was believed to have passed its peak, and many hoped it would forever disappear — in what now sounds like wishful and naive thinking.

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Elsewhere, local groups associated with Osama bin Laden’s al-Qaeda have proved similarly resilient. When the group’s affiliate in Algeria, al-Qaeda in the Maghreb, was largely quashed by the Algerian military in the populated north of the country, its militants simply moved south, joining forces with Saharan smugglers and criminals to forge an even more effective and deadly group that went on to take Westerners hostage and even seized one of Algeria’s biggest gas installations.

The Islamic State has also attracted more petty criminals than have prior militant groups, raising fears that its territorial losses could result in the emergence of new criminal or terrorist groups beyond the borders of Syria and Iraq. Taking out the Islamic State’s top leaders may have slowed down that process, but it’s unlikely to persist — at least if history can offer any indication.

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In most cases, “a leader’s death only speeds up the rise of even more radical successors,” Raffaello Pantucci, director of international security studies at Britain’s Royal United Services Institute, explained in an earlier interview.

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A good example of that is the Nigerian terrorist group Boko Haram, which gained notoriety 10 years ago with mass-casualty attacks. Authorities later arrested and killed the group’s leaders, drawing applause from victims of Boko Haram, but immediately sparking calls for revenge among its supporters.

One year later, Boko Haram militants were back, citing their leaders’ deaths for the group’s forceful resurgence. The World Bank estimates that more than 20,000 people have since died as a result of Boko Haram’s confrontation with the government, and millions have been displaced. Despite new, more recent claims by the Nigerian government that the group is defeated, attacks have not faded.

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Recent visitors to Kabul will confirm that the same can be said about the Taliban, which was declared effectively defeated only months after U.S. airstrikes and Afghan resistance forces toppled the Taliban government in November 2001, leading to an influx of U.S. and NATO troops. While the number of U.S. troops has gone up and down since, Taliban attacks have only become more frequent — and more dangerous.

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In regard to Syria, the Afghanistan experience may prompt some uncomfortable questions for politicians on both sides of the U.S. political spectrum. Has the Islamic State been defeated, as Trump claims? And if not, as Trump’s critics maintain, could U.S. troops really prevent the group’s reemergence?