The so-called Islamic State’s (ISIS) caliphate is crumbling. As the U.S.-backed Syrian Democratic Forces move deeper into Deir Ezzor province – in an area Washington refers to as the Middle Euphrates River Valley (MERV) – it will need local allies to provide security, to govern and to stabilize territory seized from ISIS.

Though much of Deir Ezzor’s local Arab population are wary of a Kurdish-dominated administration ruling over it, their alternative is to return to the rule of President Bashar al-Assad and consequently, Iran and its proxy militias. The latter would be far worse.

This tacit preference for U.S.-backed forces over Assad, however slight, is an opportunity for Washington to have a lasting impact on the future form of eastern Syria. However, this opportunity will be missed without a force of Deiris (natives of Deir Ezzor) leading security, governance and administration to build stability and reconstruct their homes and neighborhoods after ISIS is evicted.

Therein lies the challenge: Despite the best intentions of the Deir Ezzor Military Council (DMC), the organization within the SDF carrying out the military campaign against ISIS in eastern Syria, it is not up to the enormity of this task. The council is undermanned, it was recently led by a disreputable commander, and has not been given sufficient resources to prepare for the arduous stabilization mission that the U.S.-led coalition must entrust to it. Unlike in Raqqa, little effort has been made so far to establish security infrastructure, such as an interim security police force, that could help local councils to govern and bring administration and rule of law back to parts of Deir Ezzor.

This oversight will likely have catastrophic consequences for the stability of SDF- (i.e. U.S.-) controlled areas of Deir Ezzor. ISIS has shifted from being a “state” to an insurgency and will continue to undermine U.S.-backed forces and government loyalists in east Syria. In more than three years of ISIS rule over Deir Ezzor, the militant group has been working towards actively recruiting and establishing social networks on the ground. What’s more, it has been particularly focused on indoctrinating what would be the most readily available source of manpower available to the SDF: local tribal youth who have come of age under militant rule.

Without the rapid mobilization of a local fighting force to make up for the DMC’s shortfalls, there will be large governance gaps in Deir Ezzor. The U.S. will not be able trust the local population in the weeks and months following ISIS’s probable fall from power.

The militant group intends to target U.S. and SDF-supported local leaders and officials in the security, governance, administration and humanitarian fields. It will likely attempt to seed operatives into the flow of IDPs returning to SDF-run areas of Deir Ezzor, and try to infiltrate local security forces in eastern Syria that will presumably have been formed hastily without proper vetting. It is a devil’s dilemma that will be very difficult to resolve because of the rapid nature of SDF advances.

There is a solution: the return of the Deir Ezzor’s Free Syrian Army (FSA). There are roughly 3,000 displaced fighters who fought ISIS and are now eager to return to ensure neither Assad nor militants reclaim their homeland. These fighters, now spread among a dozen or so groups, are a feasible option for the U.S.-led coalition. In return for their support to the post-ISIS stabilization effort, the Deir Ezzor FSA would have the foreign sponsor it always lacked.

There is, however, one roadblock in this seemingly perfect match: the shortsightedness of Deir Ezzor’s FSA fighters. The group’s leaders’ stubborn refusal to work with the SDF is standing in the way of their return home. Pragmatism would inform them to cut a deal with the Americans and the SDF; to return to Deir Ezzor in association with, not in submission to, the SDF. Instead, leaders have refused any alliance, partly due to ethnic animosity and partly to their belief that the SDF intends to return Deir Ezzor to Assad.

Once again, just like when ISIS took over Deir Ezzor in the summer of 2014, the displaced FSA leaders have failed to understand their situation. Contrary to what they had hoped, there is currently no alternative to Assad other than the SDF. By refusing to work with the latter, they would only hasten the regime’s return.

After being displaced for three years, it is now time for the Deir Ezzor FSA’s leadership to lead. The U.S.-led coalition needs the Deir Ezzor FSA, but even more so the opposition group needs the U.S. As the SDF marches deeper into the MERV, now is their chance, and they should take it.

The views expressed in this article belong to the author and do not necessarily reflect the editorial policy of Syria Deeply.