Talk to any Syrian you meet on the Syrian-Turkish border these days, and in less than five minutes the conversation is likely to turn to Da’ash—the Arabic acronym for the rebel organization known as the Islamic State of Iraq and Greater Syria, or ISIS. Linked to al-Qaeda, the fearsome group has swept across northern Syria, imposing sharia law, detaining and even beheading Syrians who don’t conform to its purist vision of Islam, and waging war on rival militias. In early December, the group killed a foreign journalist, Iraqi cameraman Yasser Faisal al-Joumali, who was reporting in northern Syria. Even using the word Da’ash—seen as derogatory by the group’s members—is punishable by eighty lashes, a twenty-three-year-old wounded fighter from a rival Islamist group told me from his bed in a Syrian-run makeshift clinic in Turkey.

Since its appearance last April, ISIS has changed the course of the Syrian war. It has forced the mainstream Syrian opposition to fight on two fronts. It has obstructed aid getting into Syria, and news getting out. And by gaining power, it has forced the US government and its European allies to rethink their strategy of intermittent support to the moderate opposition and rhetoric calling for the ouster of Syrian president Bashar al-Assad. After months of shunning Islamist groups in Syria, the Obama administration has now said it may need to talk to the Islamist Front, a new coalition of hard-line rebel groups, in part, because they might prove a buffer against the more extreme ISIS. Ryan Crocker, a former top US State Department official in the Middle East, has told The New York Times that American officials, left with few other options, should quietly start to reengage with the Assad regime. In December, US and Britain suspended non-lethal assistance to rebel groups in northern Syria after one base fell into Islamist hands.

“Syria is now viewed as a security problem, not one about ousting Bashar and helping the Syrians get what they want,” a Western diplomat in Istanbul told me.

The influence of the Islamic State of Iraq and Greater Syria is all the more startling given how recently the group entered the conflict. Consider the eastern city of Raqqa, which was first captured by various rebel forces in early March 2013. When I visited that month, the city was ruled by a coalition of militias, and it was possible to move around as a woman without a headscarf. I met with an Alawite nurse who worked alongside Sunni peers. And I talked to Abdullah al-Khalil, a prominent lawyer before the war, who as head of the local council continued to pay street cleaners salaries and was trying to secure enough money to keep other services going.

But within two months, ISIS was firmly in charge. The group beheaded three Alawites in the city’s central square, and established sharia courts and policing. Abdullah al-Khalil, the head councilman, was himself kidnapped by ISIS or its allies. Women have been told to cover up, smoking banned, and girls and boys segregated in school. Minorities have been hounded out of the city, and foreign journalists and aid workers are no longer welcome: dozens are currently in ISIS captivity.

Mike King

In the months since its takeover of Raqqa, ISIS has quickly become one of the most powerful forces on the ground, despite its modest manpower estimated at some 7,000 fighters. It has started expanding north and west, all along the border with Turkey. In August ISIS fighters led the rebel capture of Minbegh, an airbase close to Aleppo. And the group has kicked out other rebel militias to gain control of Atmeh, al-Bab, Azaz, and Jarablus, four border towns that serve as the gateways to the outside for northern Syria and which are now known as mini-emirates. According to Syrian rebels, aid workers, and civilians I spoke to, they are using such strategic towns to control who and what can move in and out of Syria. When I visited the Turkish side of the border, trucks were lined up for miles waiting to transfer goods to other Syrian vehicles at the border: drivers are unwilling to enter the country. For their part, Syrian civilians and rebels who had just crossed from Syria into Turkey said they were terrified by ISIS checkpoints. “None of us can go in any more,” an aid worker in Antakya said.

ISIS’s spread along the border is particularly ominous for the more moderate rebel groups, loosely allied militias known as the Free Syrian Army, which have long depended on access routes from Turkey into northern Syria. 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. When discussing ISIS with Syrians at the border, I often saw arguments break out. Some claimed that ISIS was better than other less devout groups because it was less corrupt (though there is some evidence of racketeering) while local criminals with guns steal cars and occupy houses. But I found hardly anyone who supported ISIS’s extremist vision of society. Most rebels and Syrians are prepared for a second war against ISIS, though they disagree whether to do so now or—assuming that happens—“when Assad goes.”

ISIS’s rapid growth is subject to much conjecture. The most common speculation I encountered was that ISIS is a creation of Damascus, or its ally Iran, intended to fragment the opposition and ruin the revolution. “Simply, we see it as an extension of the regime,” Khaled Kamal, a sheikh from Latakia now based in Antakya said.

While there is little evidence of any direct ties to the Syrian government, it is true that Assad has done all he can encourage the impression that the rebels are foreign-sponsored “terrorists” attacking the regime. And he has helped that come about. Syrian lawyers have documented how in the early weeks of the revolt, the regime let out Islamist prisoners from Saidnaya prison—probably to foment radical Islamism within the opposition. While ISIS wages battles against the regime, including currently in the eastern province of Deir Ezzor, Aleppo, and Qalamoun, near Damascus, the goal of defeating Assad appears to be secondary to consolidating their own rule in rebel-held areas.

But Turkey, now a sworn enemy of Damascus, has also done much to allow ISIS to grow by allowing foreign jihadists to cross its border into Syria. A large majority of foreign fighters who have entered Syria come through Turkey, including many Iraqis who share their own border with the country. Since late 2012 houses in Reyhanli, a border town, have been turned into staging posts for foreigners; I have visited one. The Alice Hotel in the same town is known as something of a jihadi hangout. The plane from Istanbul is known as the jihadi express. At points foreign jihadis have been present among other groups manning the border of Bab Hawa. On my most recent trip to the border, I saw very few Turkish police. Five minutes in Kilis, a town on the Turkish side of the border north of Aleppo, was enough to spot foreign fighters hailing a taxi to the Syrian border.

A Syrian with close ties to Turkish officials told me that the Turks pass the buck: “the third countries let them leave so why should we stop them?” Last month, perhaps in a sign of the mounting pressure, Turkey reported that it had kicked out 1,100 European fighters. At points it has seemed upset at the foreign fighters, closing the border this fall when ISIS took over nearby areas. Still, Ankara seems reluctant to clamp down on ISIS in areas where it has battled the Kurdish PYD, whose growing strength is a threat to Turkey. (The PYD has close ties to the PKK, the militant Kurdish group in Turkey which Ankara is now trying to make peace with.)

On the ground, ISIS’s relations with other rebel groups often depends on the area in question and the local emirs in charge. Fights have broken out in Latakia, a northwestern province, where Abu Ayman al-Iraqi, the local emir, has made trouble, assassinating a local commander. The north and east have also seen outbreaks of violence. A lack of more clashes may simply reflect ISIS’s consolidation of power. Small groups tend to join ISIS, for protection or to avoid risking trouble. If they are big enough, there is a pragmatic stand-off.

If rebel commanders are reluctant to be openly critical of ISIS, their subordinates are less so. “They are foreigners occupying our land,” one fighter for Ahrar al-Sham, a large Salafist network, told me. “They ban people from smoking straight away—not even a doctor would prescribe that!” another rebel fighter, a nineteen-year-old from Aleppo, said. ISIS has also changed Syrians’ view of the war. “If the choice is between ISIS and Assad, I’ll take Assad,” says a Syrian friend who enthusiastically supported the protests.

As significant, if not more so, is the shift I have noticed in conversations with Western officials about the Syrian crisis in recent months. Since early September, when President Barack Obama sought, and failed to win, Congress’s approval for limited strikes following Assad’s use of chemical weapons, talk of intervention has been replaced by a growing push for a political solution. In recognition of the lack of power wielded by Western-backed fighters, American officials have recently met with the Islamic Front, the new coalition of seven rebel groups, and have said they may pursue more such talks—although the Front has so far refused.

Meanwhile, some intelligence agencies, including Germany’s, have reopened links with the Syrian government. It is possible to imagine a further rehabilitation of the Assad regime, as the al-Qaeda threat continues to grow. Some analysts have suggested the West should pursue a Sunni Awakening strategy for Syria along the Iraqi model—paying tribal Sunni militias to fight al-Qaeda—though so far there is no sign of that happening.

What is indisputable is that the Salafist-jihadist insurgency, and the emergence of one of al-Qaeda’s most fearsome affiliates within it, has fundamentally changed the war in Syria. In a conflict in which some 6,000 people continue to die every month and a third or more of the population have been forced to leave their homes, the problem of basic security has almost completely supplanted the aspirations of a once-peaceful protest movement trying to take on an autocratic, militarized, and sectarian regime. And as the regime—with its own outside support from Hezbollah and other foreign fighters—has resorted to increasingly brutal attacks, organizations like ISIS have spread unprecedented terror on the rebel side.

While Syrians continue to suffer, sandwiched between a brutal dictatorship and extremist groups, Arab and European jihadists are being indoctrinated and trained in the world’s most active battle zone—experience they may someday bring home.