The fall of the self-styled caliphate carved out by the so-called Islamic State in Syria and Iraq has been nearly as spectacular and swift as its rise.

ISIS-held cities and towns on both sides of the Syrian-Iraqi border have fallen like dominoes since Iraq announced the liberation of the northern city of Mosul in July. Iraqi forces took nearby Tal Afar at the end of August, completing that battle in less than a fortnight. And clearing ISIS from Hawijah, its last urban stronghold in Iraq, took barely a day.

In Syria, the jihadist movement is also on the run. US backed forces on Oct. 17 declared the end of military operations against ISIS militants in the eastern city of Raqqa. Presented as the ISIS “capital,” Raqqa, like Mosul, was a major hub for foreign fighters seduced by ISIS’s jihadist state-building project.

The caliphate is in ruins, the vast swaths of territory that once sat beneath its black banners dwindling by the day. Its architects are either dead, on the run, or hiding in desert outposts. Yet the demise of ISIS as a physical entity in Iraq and Syria does not spell the end of this jihadist network.

Gabriel Chaim/AP This Thursday, Oct. 19, 2017 frame grab made from drone video shows damaged buildings in Raqqa, Syria, two days after Syrian Democratic Forces said that military operations to oust the Islamic State group have ended and that their fighters have taken full control of the city.

Already it has spawned a multitude of affiliate groups, or “provinces,” across Africa, the Middle East, and Asia. But these regional branches fall along a broad spectrum in terms of the depth of their connection to ISIS, and few have their origins truly rooted in the battlefields of Iraq and Syria. Many are born of local grievances, their allegiance to ISIS bringing benefits to a cause already long established, and the barriers to any of these groups becoming the heart of a new Islamic State are high.

“I think ISIS will go from being a proto-state to an underground terrorist organization, and this organization will have affiliates in different places in the world,” says Ali Soufan, a former FBI agent closely involved in investigating the 9/11 attacks, and author of “Anatomy of Terror: From the Death of bin Laden to the Rise of the Islamic State,” who now runs The Soufan Group. “But then there’s the ideological question: What’s going to happen if there’s no caliphate? They gave allegiance to a caliph and to a state that no longer exists.”

At its peak in 2014, ISIS controlled more than 100,000 square kilometers, mostly in Iraq and Syria, an area containing some 11 million people, including up to 40,000 foreign fighters from more than 120 countries. But by early 2017, the number of people living under ISIS had already dropped by 56 percent in Syria and 83 percent in Iraq, according to a study by the RAND Corp.

Some of the fighting has been bitterly contested – witness the nine-month campaign to retake Mosul in Iraq – but with Raqqa now also out of the way there are high hopes military operations in Iraq and Syria will be concluded more quickly than anticipated.

ISIS resilience

These hopes, however, should be “very carefully caveated to be the taking back of territory and seeing the colors on the map change,” says Chris Maier, director of the Defeat ISIS Task Force at the US Department of Defense, “as opposed to the idea that there won’t still be a clandestine or insurgent-type ISIS capability in both Iraq and Syria for the foreseeable future.”

Indeed, Mr. Maier mentions an oft-used phrase in the Defeat ISIS community: “Taking back territory is necessary but not sufficient,” recognizing the fact that even if their territorial core were gone, ISIS would still be able to find “cracks and seams” in other parts of the world, particularly regions reeling from instability and poor governance.

“Looking into a crystal ball,” adds Maier, “I think you’ll still see a bit of a far-flung enterprise that’s calling itself ISIS, but is really more linked in name and brand than in any kind of cohesive operational coordination.”

Some analysts argue that ISIS saw the end coming and redirected its recruits to other parts of the world. If the so-called Islamic State is to see a rebirth elsewhere, it is likely to be a question of where fresh fighters are drawn, rather than where those already in Iraq and Syria choose to relocate.

In part, that’s because the majority of the ISIS leadership is Iraqi and so is likely to stay put. In part, it’s because many of the foreign fighters were determined to stay to the bitter end – and then the logistics of actually making such a move are daunting in the face of international efforts to cut them off from the rest of the world.

Militant website/AP/File In this photo released on April 25, 2015, by a militant website, young boys known as the "lion cubs" hold rifles and Islamic State group flags as they exercise at a training camp in Tal Afar, near Mosul, northern Iraq.

Core of believers

“What we have left today is the core of ISIS, the ones who really believe in it,” says Hassan Hassan, co-author with Michael Weiss of “ISIS: Inside The Army of Terror.” The group appears contained for now but, he warns, it could stage a come back if the political and social grievances that set the stage for their rapid ascent in Syria and Iraq remain unaddressed.

“ISIS will continue to be a threat,” says Mr. Hassan. “The chance for them to revive the caliphate is still there. They have the imagination for it, and they have the environment for it.”

Few analysts believe the caliphate project is likely to be resurrected in the short term. Nevertheless that should not belie the threat the jihadists still pose – nor indeed the danger that the remnants of ISIS itself represent, whether its fighters have returned home, travelled to other hubs of ISIS activity, or survive locally as part of an organization that morphs back into something akin to its original existence as an insurgency.

“The image people often have is plane-loads of these guys flying out,” says Daniel Byman, author of “Al Qaeda, the Islamic State, and the Global Jihadist Movement” and a senior fellow at The Brookings Institution. “But that’s the wrong image: It’s people filtering out in dribs and drabs.”

US Central Command said in April that it had identified more than 40,000 foreign fighters who joined ISIS in Syria and Iraq. Some analysts, however, believe those figures to be an exaggerated estimate, offering a more conservative range of 20,000 to 30,000 that includes supporters as well as combatants.

There is no doubt that a significant number of foreign fighters have already fled the battlefield. At least 5,600 residents or citizens of 33 countries have made it home, according to a report published by The Soufan Group Tuesday. Accurately assessing the risk posed by returnees is just one of the challenges for law enforcement officials trying to stave off ISIS attacks.

Role of sleeper cells

Lina Khatib, head of the Middle East and North Africa Program at Chatham House in London, warns that territorial losses do not necessarily translate into a reduced capacity to stage attacks worldwide.

“Military defeats in Iraq and Syria hurt ISIS symbolically more than operationally, because ISIS activities in areas outside of the Middle East and North Africa are conducted by sleeper cells – sympathizers who take orders from the ISIS leadership who are hiding in the desert between Iraq and Syria in some cases, but who operate in an opportunistic fashion in other cases,” says Ms. Khatib.

An estimated 6,000 to 10,000 ISIS fighters remain in play in Iraq and Syria.

The Soufan Group notes in its report that even as ISIS has ceded control of its territorial caliphate, concern that ISIS may remain viable in the long term, both as a group and as an inspiration, has continued, in large part because it has been so successful in attracting foreign recruits.

“People not only flocked to join it in Iraq and Syria, but have also done so in other parts of the world, whether or not (ISIS) has established a formal province there,” it noted.

Libya is one of the few places that did receive an outflow of fighters from the core of ISIS, and while its ouster from the city of Sirte at the end of last year pushed it out into desert areas, the chaotic nature of that country makes the eradication of ISIS a tough proposition.

Another area of concern is Egypt, particularly in the Sinai, where there has been a recent uptick of attacks and an apparent expansion of the ISIS presence.

In many places where ISIS seeks a foothold, it is in competition with other groups for recruits and resources. In Afghanistan, for example, where there is a robust presence – referred to by some as ISIS-Khorasan – they are under pressure from the Taliban and the Haqqani network. In Yemen and Somalia, Al Qaeda is the main competitor; in the latter, where ISIS attempted to turn the Al-Shabaab militant group, Al-Shabaab resisted and even tried to crush the ISIS presence, meaning that perhaps fewer than 100 ISIS fighters remain.

The ISIS bandwagon

This sequence of events reflects a characteristic of many of the groups that claim allegiance to ISIS: long-established insurgencies that simply jumped on the ISIS bandwagon to attract attention and boost their cause.

“The way ISIS has historically operated, and Al Qaeda too, is they don’t create anything from scratch,” says Seth Jones, director of RAND’s international security and defense policy center and a former assistant secretary of Defense for special operations. “They are leveraging existing local groups.”

This is true of West Africa, particularly the Sahel region, where four US service members recently lost their lives on a counter-terrorism assistance mission in Niger. Boko Haram has long been prosecuting an insurgency in Nigeria and pledged allegiance to ISIS, but a multinational military operation has deprived it of much of its territory.

And then there’s the Philippines, where ISIS-aligned fighters have been battling government forces in the city of Marawi for five months. While the Mindanao region has seen insurgent activity for decades, not least in the form of the Abu Sayyaf group, the explosion of violence in Marawi has taken it to new levels. It also speaks to an increasingly international trend in terms of cooperation between militant groups: the conflict in Marawi has drawn foreign fighters, though mostly from Indonesia and other parts of Southeast Asia.

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ISIS jihadists are “clearly able to embed themselves in lawless states, failed states, ungoverned border areas,” says Bruce Hoffman, director of the Center for Security Studies at Georgetown University, “so there has to be a significant international effort to deny them that.”

Dominique Soguel contributed from Basel, Switzerland.