Two months ago, the Islamic State of Iraq and the Levant (ISIS) began its push into Deir Ezzor, the eastern province bordering the extremist faction’s base in Iraq. Waiting in the well-populated corridors formed by the Khabur and Euphrates rivers were Jabhat al-Nusra, the al-Qaida-affiliated group well-entrenched with local tribes, and its sometimes allies, the Islamic Front.

At stake for ISIS in Deir Ezzor, one of the top oil-producing provinces in the country, is not just millions of dollars in smuggled oil receipts, but a strategic foothold into eastern Syria and beyond. But Nusra is strong in the region, pushing back in fierce fighting that has caused tens of thousands of civilians to flee.

Valerie Szybala is an analyst with the Institute of the Study of War who focuses on the dynamics of Deir Ezzor. Here, she weighs in on what makes the province such a valuable prospect.

Syria Deeply: What’s happening today in Deir Ezzor?

Valerie Szybala: ISIS started this offensive much earlier than people realize. They began at the end of March, slowly pushing down from Hassakeh and ar-Raqqa into Deir Ezzor, via the Khabur River and Euphrates River. The two rivers come together at al-Basirah, just south of Deir Ezzor city, making al-Basirah a linchpin and a strategic point in the province.

ISIS had been both pushed out and strategically withdrawn from Deir Ezzor in January and February when rebels rose up against it, and now it has returned with a fury. It was surprising how successful they have been. The most powerful group there has been Jabhat al-Nusra, which works in the province with Jaish al-Islam and other Islamic Front groups. This JN-rebel coalition had been in primary control of many areas. It’s possible Nusra wasn’t expecting ISIS’s resurgence, and were focused on other battles and were therefore unprepared in Deir Ezzor when ISIS decided to concentrate on it again.

Syria Deeply: Where does the government fit in here?

Szybala: It’s clear that the regime has benefited from ISIS coming back in Deir Ezzor. The regime strongholds in the province are minimal, and they really only have forces remaining in a few neighborhoods in the northern part of Deir Ezzor city and the military airbase.

All of these sites had been close to crumbling under continued rebel attacks prior to the ISIS resurgence. Also, regime airstrikes on JN and Islamic Front forces have been closely correlated with the ISIS advances, paving the way for ISIS to make new gains. One component of this offensive has been both JN and ISIS vying for support of local tribes. Deir Ezzor is a tribal region and many are from the same tribal groups that exist just over the Iraq border. Other than that it’s a largely unpopulated desert.

Syria Deeply: Which group will win?

Szybala: I don’t think either ISIS or Nusra is a natural fit to gain the tribes’ support from an ideological perspective. The allegiance of tribal groups have been bought for the most part, and most allegiances have fallen on Nusra’s side. Some of it is because ISIS is very brutal and Nusra is much friendlier to civilians, so they’re more welcome. It’s also beccause Nusra has, through the Sharia councils that it has established, been in charge of many of the oil reserves in Deir Ezzor. They’ve been able to set up deals and profit sharing agreements with local tribes, coopting their support.

Syria Deeply: Why does each side want this province so much?

Szybala: Deir Ezzor has two strategically appealing aspects for both JN and ISIS. One is oil. The province’s oil production has fallen dramatically from prewar levels, especially since the regime pulled out of most oil-producing areas in late 2012 and early 2013. Oil produced out of wells has been improved since the rebels took over its production. For a time after that it was a free-for-all, with oil being refined in small handmade facilities, which were incredibly damaging to the environment and dangerous to the people who ran them. Over time, more organized efforts such as those of the Sharia Council run by Jabhat al-Nusra have boosted production by utilizing the former oil and gas infrastructure in the province.

Some of the oil currently produced is sold in Syria, smuggled to various oil markets that can be found as far as Aleppo (which implies that it’s gone through ISIS-controlled territory in Raqqa). But most oil produced by rebel groups [is produced by Nusra] and is smuggled to southeastern Turkey, where a new black market economy has developed around it. The financial support coming out of this production makes these groups largely self-sufficient.

Vice magazine has estimated it as $500,000 to $1 million a month total illegal sales, while the Carnegie Endowment has produced an estimate of up to $50 million a month. The real number is somewhere in between. But in any case it is a pretty substantial amount of money being smuggled out of Syria, especially when you consider that much of it is going to al-Qaida-affiliated groups.

The second strategic factor in Deir Ezzor is that it is an important supply route and transit zone. For ISIS, which has its main military center of gravity in Iraq, the province connects its [stronghold in] Iraq to its stronghold in ar-Raqqa, so it is important for them to have freedom of movement in Deir Ezzor.

Also, the regime has pretty much lost control in Deir Ezzor. There are a couple of remaining enclaves that house regime forces, but it’s been essentially ungoverned at a national level for a long time. Extremist groups looking to assert control are going to gravitate there.