On September 5, the Syrian military with Russian support including cruise missile strikes, successfully broke ISIS’ three-year siege on the eastern Syrian city of Deir ez-Zor. This significant victory indicates that the Syrian Arab Army (SAA) is gearing up to reclaim that province and in doing so reestablish a sizable presence there for the first time in years. 
“The SAA has broken through to its enclave, but the real fighting for the ISIS controlled part of the city of Deir has yet to begin,” Professor Joshua Landis, a Syrian expert and head of Middle East Studies at Oklahoma University, told Rudaw English. 

“The SAA's achievement is important,” Landis went on to clarify. “It means that the enclave that has been besieged for almost three years is now relieved. It is also a major sign that ISIS is on the way out and that the Syrian government under President Assad will conquer the remaining Euphrates Valley.” 

The lifting of the siege also comes shortly after the US-backed Kurdish-led Syrian Democratic Forces (SDF) secured about 65 percent of Raqqa, ISIS's de-facto capital, following the capture of its Old City and seem poised to capture the entire city before the end of this year. Two major offensives against the militants at once could force ISIS from these two major cities and effectively deny them the opportunity to use Deir ez-Zor as a substitute capital for Raqqa. 

While Russia militarily supports the regime the Americans are supporting the SDF. These two campaigns are separate. As part of deconfliction agreements, Washington and Moscow have set clear agreed upon lines to ensure these two separate campaigns can be conducted smoothly. 

“We've established some measures south of Raqqa. And those measures extend to the east of Raqqa,” US General Stephen Townsend recently told a reporter who asked about the establishment of deconfliction zones in the Euphrates River Valley. 

While Townsend didn't go into specifics he did say that most discussions about these clear lines “are just to make sure that everybody knows what everybody's doing and not to trip on each other, really, and that the measures are still in place and we're all observing them.” 

If the regime and the SAA can muster the manpower needed to decisively rout ISIS from Deir ez-Zor in the coming weeks and months, they will likely be the ones to take it. While the SDF have voiced a desire to take that region they will likely only take and secure northern parts of that province, not major towns or Deir ez-Zor city.  

The US doesn't necessarily oppose the Syrian regime destroying ISIS. There are fears, however, that Iran could ultimately be the beneficiary of a restoration of the regime's military power in the country's east. 

“Many pundits in Washington were claiming a few months ago that the SAA didn't really exist and couldn't put together this sort of counter-attack against ISIS,” Landis stated. “They clearly didn't know what they were talking about or were trying to convince the US Department of Defense to take the Euphrates Valley.” 

“There are many in Washington who believe that both Israel and the Gulf countries will be placed in great peril should Assad and his Iranian backers be able to control the Eastern Syrian desert, forming a 'land bridge' between Iraq and Lebanon,” he elaborated. 

The Iranian land-bridge, or corridor, was first publicized in detail by Martin Chulov last year in The Guardian. Its prospect raises serious questions about who will fill the power vacuum left by ISIS in Iraq and Syria. In Syria it appears that the SAA may well be the predominant power in the Deir ez-Zor region after ISIS while the SDF will hold Raqqa, for the time being at least. In neighbouring Iraq there are indications that pro-Iranian groups in the Shiite-majority Hashd al-Shaabi paramilitaries will take control of Tal Afar, which was recaptured from ISIS by the Iraqis late last month. 

“We still do not know how much more fighting power ISIS has. Increasingly, ISIS appears to be falling apart and losing its ability to sustain major coordinated battles,” Landis concluded. “This would suggest that the fall of Deir may not take as long as Raqqa, but the proof will be in the pudding.”