When ISIS collapses what hate will replace it?

This impending victory will severely weaken this hateful organisation. But ISIS, its ideas and surviving fighters will not be eliminated completely. As the terrorist atrocity in Manchester earlier this week tragically illustrated, they will continue to pose a threat, including to Europe, for some time to come.

After months of punishing, street-by-street fighting, Iraqi government forces are steadily retaking control of Mosul, the largest city occupied by ISIS. They are being supported in this endeavour by both the US and Iran. It is rare for those two antagonists to share an objective and this illustrates just how widely the global opposition to ISIS ranges.

The pivotal moment for proclaiming the Islamic State’s overthrow will come when the ancient al-Nuri Mosque is recaptured. It was from this mosque that the leader of ISIS, Abu Bakr al-Baghdadi, proclaimed its caliphate (an Islamic state) in 2014. Meanwhile, on the Syrian side of the border, local militias backed by the West, Turkey and other regional powers are similarly rolling up the territory occupied by ISIS. They are closing in on its longer-standing headquarters, Raqqa, and it is increasingly becoming a question of when the city will fall, not if.

As well as liberating the long-suffering people living under ISIS’s brutal reign, the ending of its territorial control will be a major boost for the rest of the world. It will deprive ISIS of a substantial operating base, revenue generating resources (such as the sale of oil and ancient artefacts) and a magnet for attracting new recruits. Indeed, the military pressure it has been under has already slowed the arrival of new foreign fighters to a trickle when compared with previous years.

This success is a product of previous President Barack Obama’s strategy of letting local forces take the lead against ISIS while backing them with western air power and special forces support. As well as seizing territory, the advancing armies and air strikes are killing large numbers of ISIS fighters.

But you can’t kill them all. Plenty will survive. The locals among them, and some of those who cannot easily find a way back to their home countries, will scatter into the desert, farmland and villages around the cities and the Iraq-Syria border region. From there, they will continue to impose sporadic suffering on the people around them.

Others will venture further afield to places where ISIS already has a foothold, such as Somalia, Libya and the Sinai Peninsula of Egypt. This influx of fighters will bolster the ISIS affiliates in those locations, increasing their existing ability to violently destabilise them and inflict terrorist attacks on both local and foreign civilians. Particularly high proportions of ISIS’s foreign fighters come from Morocco and Tunisia. Returnees to those countries are another significant cause for concern.

The most immediate worry for most Europeans are the violent extremists who will make their way back to Europe. Some fighters have already returned from Iraq and Syria to commit terrorist atrocities at home, such as the November 2015 attacks in Paris.

Having several thousand more such individuals on the loose around Britain and the continent is not a happy prospect.

It is an eventuality for which ISIS appears to have been preparing. In addition to bombings like the Manchester concert killings, analysts have noted a sharp increase in its attempts to inspire solo and logistically simple mass casualty attacks in the West, such as those carried out with vehicles in Nice, Berlin and Westminster. This change in emphasis is symbolised by its main propaganda publication, ‘Dabiq’, recently being renamed ‘Rumiyah’. Dabiq is the name of a town in Syria taken back from ISIS last year. A hadith (a saying attributed to the Prophet Muhammed) prophesised that a battle against the forces of the Roman Empire would take place there and lead to Armageddon. Now the apocalypse has failed to materialise in Dabiq, the switch to Rumiyah, the Arabic word for Rome, indicates that ISIS plans to shift the focal point of its violence to Europe and the West.

Some ISIS fighters who make it home to Europe will attempt to act upon their leaders’ exhortations to commit terrorism. Others who slip quietly back into the community to lie low, and those who are picked up by the authorities and imprisoned for their crimes, will also continue to pose a threat for years to come. Even if they do not get personally involved in terrorist acts they may well prompt others to do so. The frequency with which new ISIS recruits are initially radicalised by fellow prisoners or in groups of old friends is striking.

This phenomenon is partly explained by the common characteristics of most European ISIS recruits. As the French expert on radical Islamism, Professor Olivier Roy, explains in his book ‘Jihad and Death: The Global Appeal of Islamic State’, a large majority of them are either converts or ‘born-again’ Muslims. Although the Manchester bomber may be a partial exception, very few have deep connections with mainstream Islam. They have rarely been diligent mosque attenders or participants in reputable Muslim organisations and campaigns. Indeed, most have lived highly secular lives, often including drinking, gang membership and petty crime, such as drug dealing and robbery, before suddenly turning to ISIS’s warped version of Islam.

ISIS is attractive to such disaffected and dissolute young (mostly) men because it is perceived as being the toughest gang around and presents a nihilistic challenge to all other forms of authority. It offers them a feeling of power and status. The appeal to their latent Muslim identity is relevant too. But it is seldom the main motivation.

The overthrow of ISIS’s self-proclaimed caliphate will make it look weaker, reducing its attraction to existing participants and potential recruits in Europe. But the loss of the territory it was occupying in the Middle East will not eliminate its appeal entirely. ISIS’s virulent propaganda and terrorist campaign will keep it afloat for some time yet. And even if it does diminish quicker than anticipated, its ideas will not disappear with it. ISIS itself began as an estranged faction of Al-Qaeda. It grew in prominence after its parent organisation was severely weakened by military action and the killing of its leader, Osama bin-Laden. Further mutations into new groups will almost certainly follow ISIS’s demise.

The vanquishing of the ‘Islamic State’ will be a huge step towards ISIS’s ultimate defeat. But there is no exclusively military solution to the terrorist threat posed by the numerous violent extremists who will head home to Europe as a consequence. The police and security services have already foiled many plots over recent years. Sadly, as we saw in Manchester, a 100% success rate is probably unachievable. But smart, well-resourced intelligence and law enforcement work will continue to be crucial. This includes continuing to improve pan-European anti-terrorist coordination and minimising the harm done to it by Brexit.

In the longer term, the concept of prevention being better than the cure has rarely been more apposite. Strict segregation of known radicals and recruiters in prisons is one practical step that should be used more readily. On the outside, previous attempts to prevent radicalisation have been only patchily successful. These programmes have usually focused on the centrality of Islam to the problem. Professor Roy’s research shows that most of the European citizens attracted to ISIS share certain common characteristics. Its appeal is based more on ultra-rebellion and a desire to feel powerful than religious conviction. His findings warrant greater consideration as part of our efforts to avert further attacks.

• Paul Knott is a writer on international politics. He spent 20 years as a British diplomat, with postings to Romania, Dubai, Uzbekistan, Ukraine, Russia and the European Union in Brussels