As the Islamic State (ISIS) is driven out of its bases and strongholds in Iraq and Syria, questions arise as to what will happen to the power vacuum the group leaves behind and who might fill its lost territories. The main question is: would the defeat of ISIS strengthen others, such as Iran, for example?

The 94-year-old former US Secretary of State Henry Kissinger pondered this month what will happen to former ISIS territories in a piece published in CapX. 

“Most non-ISIS powers – including Shia Iran and the leading Sunni states – agree on the need to destroy it,” he wrote. “But which entity is supposed to inherit its territory? A coalition of Sunnis? Or a sphere of influence dominated by Iran?” 

Kissinger says the “answer is elusive” given the fact that Russia and the United States support different sides. He then argues that if this territory falls under the control of Iran's Islamic Revolutionary Guards (IRGC), or Shiite militia forces sympathetic to it, “the result could be a territorial belt reaching from Tehran to Beirut, which could mark the emergence of an Iranian radical empire.” 

The former secretary of state's warning of an Iranian territorial belt stretching through Iraq and Syria echoes that of Martin Chulov, who made the case in a series of articles published in The Guardian. In the first of these articles, published last October, Chulov reported that part of this land corridor extended through Syrian Kurdish territories under the noses of the authorities there. In a follow-up article, published in May, he wrote that Iran had to alter this route by moving it 140 miles south of these Syrian Kurdish territories, given the build-up of United States military forces there supporting the Kurdish-led Syrian Democratic Forces (SDF) against ISIS. 

Kissinger's article also came shortly before a somewhat dubious report claimed that the Syrian Kurdish People's Protection Units (YPG), the leading group in the SDF coalition, were asked by the US to deter the Syrian regime's advance against ISIS in the eastern province of Deir Ezzor on the border with Iraq. 

As with Raqqa the SDF have set up a military council for Deir Ezzor to administer the province if they ever capture it from ISIS. If they do successfully capture large parts of that province from ISIS then they will control most of the territory the militants captured in Syria. 

Any Iranian land corridor through Iraq could also be assisted by groups within the Iraqi Hashd al-Shaabi paramilitaries allied closely to Iran. Former Iraqi Prime Minister Nouri al-Maliki, a staunch supporter of the Hashd, suggested in January that the Hashd could cross the border into Syria to assist Damascus against ISIS. More generally Shiite paramilitaries in Iraq have been traveling to Syria for years to fight on the side of Syrian President Bashar al-Assad.

The SDF are on record saying they will prevent the Hashd from crossing into territories they control. Salih Muslim, the PYD's co-chair, has also said that the regimes in both Iran and Syria are hostile to the “fundamental rights” of Kurds. 

The ruling Syrian Kurdish Democratic Union Party (PYD) advocates a decentralized Syrian state and the replacement of the current regime. With the exception of a few clashes with pro-regime militias holed up in enclaves in the Kurdish cities of Hasakah and Qamishli the PYD/YPG have not fought against regime forces, instead focusing their efforts on consolidating territories under their control and fighting ISIS. 

The PYD have also said that the future of Raqqa will be determined by its residents. The party's co-chair Salih Muslim has in the past suggested that it could be annexed into the unrecognized federal region his party has set up in northeastern Syria. The US doesn't want the city to be handed over to Damascus after ISIS is forced out. However, one US official quoted by Wall Street Journal in May said the regime has “a natural home-field advantage” before predicting that, “We won't be in Raqqa in 2020, but the regime will be there.” 

For now the growing vacuum left by ISIS's collapsing caliphate in Syria will likely be filled by the SDF/YPG in Raqqa and the regime in Deir Ezzor. Pro-regime forces made an abortive attempt to capture swaths of Raqqa province in June 2016, which resulted in a humiliating defeat for them. Sapped of manpower from years of war they may face similar difficulty in retaking Deir Ezzor.

The Americans and the Russians have put in place communications mechanisms to ensure they can effectively conduct these two separate campaigns against ISIS without clashing. Depending on the momentum of both campaigns the SDF may seize any future opportunity to capture swaths of Deir Ezzor after the Raqqa operation if the regime campaign fails to recapture and hold major parts of that province. 

Last year there was talk of a race for Raqqa which never really happened, the aforementioned regime offensive failed completely after a month and the SDF's advance into the northern Raqqa countryside was in reality a preparation for the Manbij operation, which took all summer to complete. 

Upon the completion of the Raqqa operation there may well be a race for Deir Ezzor to fill as much of the vacuum left by ISIS's demise as quickly as possible.