Sarah Birke describes the growth in power of the Islamic State of Iraq and Greater Syria, ISIS, which many Syrians now regard as a foreign occupier.

A year ago, the main groups fighting on the rebel side were disorganized and badly behaved, but most of them still identified—at least in their core aims of toppling Assad and building a nation state open to all Syrians—with the street movement that started in 2011. And while Salafist-Islamist rebel groups began taking a larger part in the conflict in 2012, most of them were Syrian and viewed as part of the communities in which they established themselves.

In contrast, ISIS is a group with an international profile and an extremist view of Islamic rule. And it has shown its readiness to take on any Syrians it doesn’t like, whether opposition or regime supporters. In September ISIS ousted the moderately Islamist Ahfad al-Rasoul from Raqqa by using suicide bombings (Jabhat al-Nusra, another al-Qaeda offshoot, had clashed with the group, but had not gone this far). It pushed out Northern Storm, a local rebel band, from the town of Azaz, a staging post between Aleppo and the Turkish border. And it’s also been fighting the armed wing of Syria’s Kurdish party, the PYD, in the northeast. All of which has left little doubt about its strength, or the damage it has caused to the rebellion itself.

The mainstream opposition is in a tricky position. On December 19, its exiled leadership council, the Syrian National Coalition, issued a blunt statement accusing ISIS of “abducting people for not abiding by their self-imposed regulations” and declaring that “the Coalition does not consider ISIS a part of the opposition. Its actions serve the regime’s interests.” But the Coalition has wavered on other groups with extreme views, since disavowing them highlights the lack of fighters allied with it on the ground. For example, it denounced the US’s designation of Jabhat al-Nusra as a terrorist group in 2012 and today has an unclear relationship with other Islamist groups.

ISIS originated as an Iraq-based al-Qaeda affiliate, al-Qaeda in Iraq. The organization is led by Abu Bakr al-Baghdadi, an ambitious Iraqi extremist who has overseen relentless attacks in Iraq, causing civilian casualties, and who was designated a Global Terrorist by the US State Department in October 2011, with a $10 million bounty on his head. As the war in Syria progressed, al-Baghdadi saw an opportunity for al-Qaeda, and in January 2012, sent some footmen to found Jabhat al-Nusra with the aim of creating a new transnational state ruled by sharia law and a belief in using violence to get there.

Over the following year Nusra steadily gained strength, and in April 2013 al-Baghdadi decided it was time to merge Nusra with al-Qaeda in Iraq, expanding the geographical spread of the organization, which doesn’t recognize national borders but seeks to unite the entire umma, or Muslim community of believers, under one rule. He declared the two branches would be known as the Islamic State of Iraq and al-Sham. Al-Sham refers to Greater Syria, the whole expanse of the Levant that holds a special place in jihadist thought for being the heart of the region and close to Jerusalem. But Jabhat al-Nusra’s leader Mohammed al-Jolani, who is Syrian, refused the merger, possibly because it had not been sanctioned by al-Qaeda’s chief, Ayman Zawahiri, who later ruled that the two groups should remain separate (a ruling ignored by the ambitious Baghdadi, leading some to consider ISIS outside al-Qaeda).

In fact, while ISIS and Nusra share many aims, and both are well funded and trained, there are significant differences between the two groups. Jabhat al-Nusra stresses the fight against Assad, while ISIS tends to be more focused on establishing its own rule on conquered territory. Nusra has pursued a strategy of slowly building support for an Islamic state, while ISIS is far more ruthless, carrying out sectarian attacks and imposing sharia law immediately. And while Nusra, despite its large contingent of foreign fighters, is seen as a home-grown problem, Syrians at the border frequently described Da’ash as foreign “occupiers” in their country.

In its active online media presence ISIS, like some other groups, portrays itself as a social movement with an armed wing rather than a mere rebel group. “They are there for a political reason: to lay the groundwork for a caliphate,” Charles Lister, an analyst of Syria’s rebels, told me. In recent weeks ISIS’s attacks in Iraq have increased, making it the bloodiest period since 2008. Much of its activity has focused on the western provinces adjacent to eastern Syria, a stronghold for the group.

ISIS’s vision is phenomenally popular with hardline jihadists and their supporters—more so than Jabhat al-Nusra’s—which helps explain why the conflict has managed to attract so many foreign fighters. Fundraising campaigns on Twitter by such figures as the Kuwaiti Sheikh Hajjaj al-Ajmi indicate that significant money is coming to ISIS from private donors in the Gulf. And on every trip I have made to the Turkish towns along the border with Syria in the last two years, I have come across foreign fighters heading to fight. Many of them in recent months are coming to join ISIS.

Some analysts have argued that ISIS has learned from its experience in Iraq where Sunni tribes, communities, and fellow insurgents turned against al-Qaeda, leading to the Awakenings, when tribes, funded by the US, began fighting the group. In areas of Syria where it has gained control, ISIS has begun increasing outreach to the local communities. It has just launched a newspaper in northern Syria. Videos the have posted on Twitter show tug-of-war events or festivals in village squares after Friday prayers, often packed with enthusiastic-seeming young men. In Raqqa, the group has been handing out stickers for buses telling women how to dress. Children have been a special focus. Purple gift bags have gone to girls in some rebel-held areas near Damascus, an area where the group is gradually expanding. It has ensured a food supply in towns it controls, often pushing out any other providers so as to make the population dependent on it alone.

But ISIS’s real power comes from the fear it seeks and manages to inspire. The group has shown zero tolerance for political dissent. Many Syrians I met along the border mentioned with horror ISIS’s execution of two young boys in Aleppo due to alleged heresy. The kidnappings of local activists and journalists has deterred dissent while also whipping up anti-ISIS sentiment. The group has blown up Shiite shrines, but has also shown few qualms about Sunni civilians getting killed in the process. Beheadings have become common. Father Paolo dall’Oglio, an Italian Jesuit priest who has lived in Syria for thirty years, and who campaigns for inter-religious tolerance, is missing, abducted by ISIS during a visit to the city of Raqqa in late July. As with dozens of others who remain in captivity, ISIS has not demanded ransom or announced his execution; rather it appears to be holding hostages as an insurance against attacks.

This has caused many Syrians to despise ISIS. Since June, there have been anti-ISIS protests in Raqqa—something which requires courage given ISIS’s ruthlessness. More recently, even Islamist activists such as Hadi al-Abdullah, a prominent Syrian from Homs, have criticized the group, describing them as “Dawlet al-Baghdadi,” or Baghdadi’s state, echoing “Suria al-Assad”, Assad’s Syria, the way regime supporters refer to the country. And yet ISIS continues to recruit Syrian fighters. Some say that Syrians joined because the group offers better money and protection than other rebel outfits. In an interview posted to YouTube, Saddam al-Jamal, a former leader of Ahfad al-Rasoul, explains that he defected to ISIS, because moderate fighters are subject to too much foreign interference and are pressured to fight Islamists as well as the regime.

His view is symptomatic of how hostile many Syrians have become to outside powers, which, according to many opposition supporters, have done more harm than good by supporting the opposition just enough to continue the war, but not enough to ensure a decisive victory.