By late 2017, the territory controlled by Islamic State (IS) in Iraq and Syria shrank to a tiny fraction of what it was a mere two years earlier. There is no doubt that this reversal has been a significant setback for IS, constraining the group’s ability to organise attacks overseas and limiting its appeal to potential new members. But the destruction of the caliphate should not be confused with the defeat of IS itself, let alone the end of Islamist terrorism in the region or internationally. Instead, we can expect to see a further evolution of IS and Islamist terrorism more broadly.

The terrorist organisations that have emerged in recent decades have shown themselves to be highly adaptable. IS in particular has always had a hybrid quality. In contrast to al-Qaeda-linked groups, IS aspired to establish itself as a statelike entity, trying to capture and hold territory through rather conventional military operations. But IS has also operated as a conventional terrorist group, launching attacks against civilian targets that were designed to generate fear and achieve a political goal as with the operations it has organised in Europe. The loss of the caliphate is likely to cause IS to shift further in this direction, at least in the short term.

But this is not the first time that the organisation has lost control of territory where it had established a form of public administration. IS’ history already provides us with an example of another such shift. The Islamic State in Iraq (ISI, a precursor to IS) was able to gain control of significant parts of Iraq in the years after the US invasion in 2003. It was forced to abandon the territory it held, however, following a campaign against it by a coalition of local tribes and the US and Iraqi armies. After that defeat, ISI went underground, only to reemerge stronger than ever a few years later in the space created by the civil war in Syria and the sectarian approach of the Iraqi government. There are already signs that the remnants of IS in Iraq are going underground once again, and at the same time are shifting towards terrorist attacks designed to kill people rather than gain territory. For example, IS carried out a twin suicide attack in Baghdad in January, killing dozens of people. In Syria, IS fighters have withdrawn from the major cities but remain present in more remote parts of the country. The same pattern is visible among IS’ so-called provinces in other countries. In Libya, IS lost control of its base in Sirte as well as a major camp at Sabratha near the Libyan border, but its fighters have continued to mount attacks against targets such as the oil sector and a courthouse in Misrata. IS has also carried out highly destructive attacks in Afghanistan and Egypt in recent months.

In Iraq, Syria and Libya, IS has undoubtedly been weakened, but it could nevertheless regroup and rebuild in the future if it gets the chance. These countries remain divided by conflict, political breakdown and sectarian divisions, factors that could create space for IS to re-establish a foothold. The retreat of IS could also play into the hands of other Islamist extremist groups, notably those attached to al-Qaeda, which have entrenched themselves by forging local alliances.

Although al-Qaeda’s affiliate in Syria has split off from the central organisation, it remains an influential force on the ground. Groups linked to al-Qaeda also remain present in Yemen, Somalia and the Sahel. These groups continue to pose a threat to the regions in which they are based, though they appear less focused on orchestrating attacks further abroad at the moment.

The most pressing question about IS is what will happen to its many fighters, in particular those who travelled from abroad to join its ranks. It’s estimated that as many as 40,000 people from 100 countries around the world flocked to fight with jihadist groups in Syria and Iraq, and the overwhelming majority attached themselves to IS. In addition, as many as 3,000 foreign fighters are thought to have joined IS in Libya, though some may have travelled there from the Iraqi and Syrian theatres.

Although IS has lost almost all of its territory, the numbers of fighters who have been captured remains comparatively low. Security officials in Western countries believe that many fighters in Syria have slipped through the ranks of the Syrian army and other forces opposing IS and have made their way to other parts of the country. US military officers have been quoted in the press as suggesting that the number of fighters who have evaded capture and remain at large could be in the low thousands. Some of these fighters may be biding their time within Syria and waiting for future direction from IS’ underground leadership. Others may have joined up with the former Syrian alQaeda affiliate, Hayat Tahrir al-Sham. Others, principally foreign fighters, are thought to be trying to leave Syria, either to return home or to travel to other theatres of conflict.

Details about the number of foreign fighters who have returned to their countries of origin are often vague. Out of the estimated 5,000 European citizens who joined IS and other jihadi groups, around 1,500 are thought to have returned. In general, the rate of returns in recent months has been lower than expected. As Western countries have toughened their responses to returning fighters, more appear to be staying away. French and British officials have recently suggested that they would rather kill their citizens in Syria and Iraq than allow them to escape and return. This should, however, probably be taken as a kind of rhetorical bravado: European governments do not have the degree of precise knowledge of the location of fighters from their countries necessary to implement a shoot-to-kill policy. Instead, it is likely that we will continue to see a steady trickle of returns.

There continues to be widespread dispute within European countries about how returning fighters should be handled. After a series of incidents in which returnees were involved in attacks (including the Paris attacks of November 2015, and the Manchester attack of May 2017), authorities have tended to adopt an approach that favours the prosecution and imprisonment of foreign fighters. But it is important to distinguish between fighters who have been sent back to their countries in order to carry out attacks, and those who are returning because they cannot stay in the caliphate and may not want to adopt the life of a roving jihadist. The military defeat of IS on the ground in Iraq, Syria and Libya appears to have reduced the group’s ability to plan and carry out external operations. Therefore it has become less likely that returning fighters are coming back as part of a preconceived plan to carry out a specific attack.

Nevertheless, some of those returning will retain a commitment to the cause of jihadist terrorism and will have the operational experience to carry out further acts of violence. The challenge for Western authorities is to find a way of distinguishing among the degrees of risk that returnees pose. Locking up all foreign fighters is a short-term solution (many will face limited sentences, because they cannot be linked to any specific terrorist operation). That strategy also runs the risk of merely entrenching the alienation from society that fighters already feel and deferring any attempt to reintegrate them. Then there is the additional problem of women who adopted IS’ ideology but did not travel with the intent of fighting and the children they took with them or gave birth to in the caliphate. These are issues that demand a more precise understanding of the processes of radicalisation and de-radicalisation, another facet of the problems caused by the wave of support for jihadist ideology that has not yet exhausted itself in societies around the world.

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