The West might believe that Mosul and Raqqa are the final frontiers in the battle against the Islamic State (ISIS), but the terrorist group has, over the last six months, pulled resources and people from those frontlines to fortify the lesser-known province of Deir ez-Zor. Located in Syria’s southeast near the Iraqi border, Deir ez-Zor has been largely under ISIS control since 2014.

Conveniently located between Raqqa and Mosul, Deir ez-Zor is strategically positioned to serve as a military and supply hub for ISIS. Flanked by mountains and divided by the enormous Euphrates river, the town is a natural fortress, which will make it more difficult for ground troops to launch a surprise attack, and airstrikes alone may not be very effective.

Deir ez-Zor also holds the richest oil supply in all of Syria, which could help ISIS recover financially after losing nearly 30 percent in oil revenues and taxes from lost territory as a result of the U.S.-led offensive. And although the U.S.-led coalition had previously tried to stop oil production and trade in Deir ez-Zor, it limited its strikes to oil trucks since destroying oil fields in the region would have caused uncontrollable fires and catastrophic ecological destruction.

On top of the geographical advantages, ISIS enjoys the support of several local Iraqi-based tribes, such as those based in al Qaem, Rawah, or Anah, thanks to its clever manipulation of local tribal relations. Because several tribes in the city are separated by the Iraqi–Syrian border, ISIS effectively abolished the border and then reorganized Deir ez-Zor into two states. One state contains the cities of Mayadeen and Deir ez-Zor, the province’s namesake, as well as the northern sector, while the other covers Abu Kamal city, the southern sector, and incorporates Iraqi cities such as al Qaem. These strategies helped empower local tribal leaders who became rulers of new swaths of territories and who then threw their support behind ISIS.

In short, Deir ez-Zor is an ideal location for ISIS to recover and rebuild after its losses in Mosul and Raqqa. And it may be that when Mosul and Raqqa are reclaimed, coalition fighters will still have to reckon with ISIS’ new stronghold.

AN UNWELCOME SIDE EFFECT

Since the beginning of the operation to retake Mosul, Deir ez-Zor has seen a serious influx of ISIS fighters and their families from both Iraq and Syria. According to a local taxi driver formerly from Mosul, all ISIS fighters with the money to do so left Mosul even before the U.S.-led operation began. And by the time coalition strikes hit Raqqa and northern Aleppo, the population of Deir ez-Zor had skyrocketed. Among the new residents are the families of leaders from ISIS’ governing offices (or “diwans”), many of them foreign.

According to a doctor in a local hospital in Deir ez-Zor, many of her new female patients are the wives of ISIS foreign fighters. “I’ve started to notice that many women who are coming don’t speak Arabic,” she told us. “And if they do, they speak in Fusha [standard Arabic],” as opposed to colloquial Arabic, which meant that Arabic was not their native language and that they had learned it later in life. There have also been other indications that suggest that these women are foreign. “They also have tattoos and piercings on their bodies,” said the doctor, which would be unusual for a local.

Not all Deir ez-Zor locals have welcomed the influx of new ISIS fighters. Food prices have risen tremendously, with demand outpacing the supply of such goods. “Prices are so high lately I can’t afford living here anymore,” explained an engineer from Mayadeen city who has family in Europe. “Two years ago my brother was sending me $100 a month, and it was enough for my family. Now he is sending $220, and it is not enough even for me alone.”

But to cultivate local support, ISIS has invested in the town’s public services. Using money from oil, trade, and taxes, the militant group increased the salaries of public servants to encourage locals to get more involved in the civil service. It has also increased supplies for the hospitals it has built and has opened at least two new schools for the children of ISIS fighters. Civilian schools also exist in the town, but are not popular among the locals who prefer ISIS’ curriculum and send their children to train militarily under ISIS. Many doctors and teachers who were either supporting ISIS or were forced to work for the terrorist group have moved from Iraq to help. As one local resident explained to us, “some teachers in Hajeen”—an area in Iraq where families of ISIS fighters live—“decided to open one room in a house for a school and started teaching in their language, for example in Turkish.”

ISIS is also increasing its military presence in Deir ez-Zor. It has begun building trenches on the northern side of the province with the help of prison labor. It has also moved the majority of its weapons, artillery, and cars in Palmyra to the region since Mosul is now inaccessible, given that many of the bridges across the Euphrates have been taken out by coalition airstrikes.

This increase in fighters and supplies to Deir ez-Zor has already led to major military victories for the group. On January 17, ISIS took control of the road linking the military airport in Deir ez-Zor to ISIS-controlled neighborhoods in the city of Deir ez-Zor, strengthening the group’s position in the town.

If the dynamics in Deir ez-Zor are any indication, there will surely be another battle for control of the territory. But it will not be an easy one. For one, it is not clear who could even fight ISIS there. Although the Syrian Democratic Forces (SDF), an umbrella group fighting ISIS in the country’s east, are the closest in proximity, there is strong animosity between the locals and the Kurds who constitute the biggest proportion of the SDF. Any advance of Kurdish troops could push locals toward supporting ISIS. And although other groups, such as Fatah al Sham (formerly al Nusra Front, an al Qaeda affiliate) and the Syrian rebel group Jaysh Osood Alsharqeyyeh (Lions of the East Army) have promised to fight ISIS in Deir ez-Zor, it is not clear how successful they would be or how quickly they could move toward ISIS’ new sanctuary from Homs and Idlib, where they are based.

What is clear is that the fight against ISIS is not over. If ISIS is not deprived of its new safe haven in Der ez-Zor, it will have a new stronghold to launch insurgency activities and cause more destabilization in the region.