The first signs of a Western-backed attempt to recapture Raqqa, ­Islamic State’s de facto capital in Syria, came a fortnight ago when fighter jets dropped leaflets over the city telling residents to leave. “The time has come,” the warnings read, alongside an illustration of residents evacuating the city as incoming forces overran IS fighters.

Although up to half of Raqqa’s residents fled when IS first took control of the city in 2014, the militants have made it ­increasingly difficult for the people who stayed behind to leave. Following the US-led coalition’s warnings of an impending attack, however, the jihadis relaxed their restrictions on movement. Citizens were allowed to disperse into the nearby countryside. The idea was to spare them whatever onslaught was planned against Raqqa while keeping them within IS territory.

Ever since the latest offensive against IS began in Syria and Iraq in late May, it has become clear that the group will not concede territory easily around Raqqa – or elsewhere. It might lose small villages from time to time, but all of its major urban centres remain well fortified. Few observers expect them to fall any day soon. IS has too much invested in Raqqa, as well as Mosul in Iraq. Occupying the cities fuels the group’s prestige by projecting the impression of ­viable statehood and by allowing it to house fighters and military equipment.

Raqqa is the nerve centre of IS operations. Several training camps are located on its outskirts, including those used to plan attacks against the West. IS has long anti­cipated a revanchist campaign against its Syrian base and has fortified the city by surrounding it with trenches and landmines to thwart any hostile advance.

What makes the fight against IS even more challenging is that its fighters are not easily disheartened. Before this latest campaign, I spoke by Skype to a British fighter from High Wycombe, Buckinghamshire, about how the group perceives territorial losses. He responded with the kind of fatalistic indifference that only the faithful enjoy. Their obligation, he told me, was simply to try their best. The challenge for them was to fight with all they have. Results come from Allah, so, if defeat and setbacks follow, then it is the will of God.

There are two possible interpretations, in their reasoning, for why God might not deliver success for them – because He is punishing or testing them. Either way, the conclusion is the same: to double down on their commitment. In that spirit, they are resolved to fight until victory or martyrdom – and both outcomes represent success. This reasoning shows just how hard it can be to erode the morale of IS’s most doctrinaire fighters (though not all are so zealous in their commitment).

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The ground push for Raqqa has been overseen by the Syrian Democratic Forces (SDF), which are led principally by the YPG, an ethnically Kurdish unit of fighters concentrated in north-eastern Syria. Although the SDF officially claims to be an umbrella movement for more than 20 different fighting groups – some of which are Arab – its heavily Kurdish composition has made it a reluctant and unsuitable partner in the push to liberate Raqqa.

To understand the reasons why, it is necessary to parse the conflict into its constituent parts. We often hear about the sectarian dimensions of the Syrian civil war, yet this is just one aspect of a much broader tapestry. Syria is a series of wars within a war. Just as there are sectarian components, there are strong ethnic dimensions, too. These are especially pronounced in the northern regions where the Kurds, with their cultural and linguistic distinctiveness, stand apart from their Arab neighbours.

The Kurds have usually formed defensive fighting units in the Syrian conflict, preferring to safeguard and administer their own areas rather than acquire new territory such as Raqqa. Another issue is that Arab ­civilians are reluctant to have non-Arabs push into their cities. The anti-IS activist group Raqqa Is Being Slaughtered Silently (RBSS) says that residents worry about ethnic retribution against an Arab population that is seen as having historically oppressed the Kurds. Many reason that it is better to keep IS and deal with the devil they know.

Those fears are not unfounded. With the horrors of IS and the Syrian army so magnified, it is easy to forget that every fighting group in this conflict has violated human rights and continues to do so. The Kurds are no exception; in October, Amnesty ­International accused Kurdish fighters of war crimes after they razed Arab villages in al-Hasakah and al-Raqqa Governorates. All of this adds to the intractability of the war, forcing people to seek security within their communal, sectarian or ethnic circles. Syrians are hardly unique in this respect; they are merely repeating a pattern of countless conflicts around the world.

This makes it extremely difficult for the West, which is reliant on local forces to do the fighting. The US is supporting al-Hashd al-Shaabi (meaning “popular mobilisation committee”), a nominally Iraqi force leading the assault against IS in Fallujah. Iraq’s prime minister, Haider al-Abadi, has made two main claims about al-Hashd al-Shaabi: that it is a non-sectarian movement of ordinary Iraqis from all sections of society who want to drive IS from the country, and that its leadership reports to him personally.

Neither of these claims is accurate. It is true that some divisions of al-Hashd al-Shaabi comprise Sunni fighters, but it is overwhelmingly dominated by Shias. Its military campaigns are directed not from Baghdad, but Tehran. These efforts are overseen by Qasem Soleimani, a celebrated Iranian major general in the elite Quds Force, who is perhaps the most important military official with a battlefield presence in Syria and Iraq. He previously orchestrated several successful campaigns for President Bashar al-Assad and the al-Abadi force.

Though the ongoing assaults on Raqqa and Fallujah have put IS under pressure on two fronts, anyone hoping this might signal a turning point in the conflict is likely to be disappointed. For every push that shunts IS backwards – often only marginally – many new recruits are spawned.

Videos released on social media from the latest assault on Fallujah appear to show how incoming Shia fighters have beaten and tortured Sunni civilians. The pictures of abuse are overlaid with sectarian slurs, often invoking sensitive points of disputed Sunni/Shia theology. These resound across the region and arguably do even more damage than the images of abuse.

The rapid deterioration in sectarian relations that followed the 2003 invasion of Iraq explains how IS was able to capture Sunni areas of Iraq with such ease. Ordinary residents do not necessarily agree with the authoritarian strictures of its regime, but they mostly understand them. These latest outrages from incoming al-Hashd al-Shaabi fighters will only fuel the belief among Sunnis that they are best served by Sunni administrations – however brutal.

Islamic State has repeatedly invoked the vulnerability of the Sunnis across the Levant to justify its violence. This is the constituency in whose name it claims to act and whose interests it claims to defend.

Shortly after IS first captured Mosul, in June 2014, the group released a video, aimed at Iraqi Sunnis, explaining how both the West and Iraqi Shias had conspired against them in 2003. The result had been a decline in Sunni fortunes and increased insecurity as Shia death squads sought revenge after decades of repression and abuse.

This resonated strongly with Sunnis such as the Albu Mahal and al-Qa’im tribes, which had supported the US-led “awakening”, a military strategy initiated in 2005 to encourage Sunni Iraqi tribes to fight against the insurgency initiated by al-Qaeda. IS captured the heads of those tribes and forgave them for fighting alongside the West against al-Qaeda in Iraq. We are not accustomed to seeing videos of IS pardoning captives, but this was as careful and calculated as any of its ultra-violent theatre. The exercise was designed to project the group as a bastion of Sunni honour and security.

That is the story behind so much of IS’s strength today: the fears of the vulnerable Sunni poor over whom militants govern. Remove that constituency, and the group would collapse. But the Obama administration has done little to allay Sunni fears. Rather, it has exacerbated them by launching air strikes against IS targets in Fallujah, fuelling a perception that it is working hand-in-glove with Shia militias loyal to Iran.

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The latest attempt to seize IS terri­tory points towards a more pressing question: what, actually, is Islamic State? During a recent meeting at the Foreign and Commonwealth Office, one analyst brilliantly described the mercurial nature of the group. To residents of Raqqa, it appears as a proto-state, replete with all the nomenclature of statehood: an executive, judiciary, police force and civil administration. To rebel groups in the north and for President Assad in Syria, it is more of an aggressive insurgent movement with which there are periodic battles for control of land. For the French and Belgians, it feels more like a conventional terrorist group that deploys suicide bombers and gunmen to kill as many civilians as possible.

Such is the kaleidoscopic nature of IS that there is no reason why it cannot assume multiple forms at the same time, or why it can’t move from one to the other. If the territory in which it operates is overrun, it will revert to being a conventional terrorist movement that unleashes waves of attacks against the West and others. IS has already demonstrated both its willingness and ability to strike in Europe, Egypt, Lebanon, Saudi Arabia, Tunisia and Turkey.

It now also appears an American man, Omar Mateen, self-identified with Islamic State and affiliated himself to the group in order to carry out the most deadly act of US domestic terrorism since the 11 September 2001 attacks. Mateen killed 49 revellers, and injured more than another 50, at a gay bar in Orlando, Florida, on 12 June. The ability of individuals to align themselves with IS despite having no tangible links to it underscores the difficulties of acting decisively against the group. Indeed, this is precisely what IS has advocated. A few days ago, its official spokesman, Abu Muhammad al-Adnani, repeated his call for individuals to launch attacks in the West on the group’s behalf. Following the Orlando massacre, supporters have already suggested copycat attacks in Paris, London and Washington.

By way of comparison, let’s consider what al-Qaeda looked like on the day after the 9/11 attacks. What the West faced was a small group – of perhaps 500 key individuals, if we’re generous – committed to its programme of global jihad. By contrast, even conservative estimates today place ­Islamic State’s manpower somewhere in excess of 20,000. And no one has yet convincingly addressed how to mitigate the threats that will emerge from the region should IS suffer a sudden loss of territory.

IS’s control of large parts of Syria and Iraq will not end quickly. Not only is the group embedded and emboldened, but it enjoys the strategic advantage that comes with being able to operate across two (however nominally) sovereign states. In that respect, the Syrian and Iraqi crises embody all the difficulties of the last hyphenated conflict of the past decade, the so-called challenge of “Af-Pak” (Afghanistan and Pakistan). There, the US found that whenever it pushed against Taliban fighters in Afghanistan, they disappeared over the border. When Pakistan did the same, insurgents moved the other way.

Many of the same issues undermine Western-backed attempts to eradicate IS today. When it allowed civilians to move from Raqqa into the countryside, its own families, fighters and supporters were moved

as well. It has also begun moving critical personnel and heavy arms out of Raqqa, repositioning them near the Iraqi border. In the unlikely event that its operations in Syria are severely compromised, it will fall back into its Iraqi hideouts, and vice versa.

Pressuring IS, therefore, is like squeezing the air in a balloon: push on one area and it moves to another place. In Syria, even as IS militants fight to defend their territory in Raqqa, they have made gains in the ­Aleppo Governorate, moving ever closer to the strategic town of Azaz. Whoever controls Azaz also controls the nearby Bab al-Salam border crossing with Turkey, an important source of revenue and influence. IS previously occupied Azaz but abandoned it in 2014 to consolidate its control in Raqqa. That the group is close to recapturing Azaz at a time when the Obama administration wants to suggest that IS faces an existential crisis shows just how fissiparous and ­intractable this conflict remains.

Shiraz Maher is an NS contributing writer and the deputy director of the International Centre for the Study of Radicalisation at King’s College London. His book, “Salafi-Jihadism: the History of an Idea”, is newly published by C Hurst & Co