Washington’s new allies in the fight against the Islamic State are gaining ground. But their Kurdish leaders are getting in the way.

On Oct. 10, a coalition of 13 Kurdish and Arab fighting factions from northeastern Syria formed the Syrian Democratic Forces (SDF) and emerged as the centerpiece of the U.S.-led military effort against the Islamic State, also known as ISIS, in the country. The SDF, in which the Kurds are the dominant force, brings together most of the groups responsible for the Islamic State’s most significant setback in over a year — the capture of Tal Abyad in northeastern Syria in June. The taking of that border city deprived the jihadi group of a vital gateway from Turkey and brought some of its worst enemies — Kurdish and Arab tribal fighters — within 50 miles of its stronghold in Raqqa.

The alliance has gained even more relevance with the recent offensive by Iraqi Kurdish forces to retake Mount Sinjar, an area near the Syrian territory where the SDF operates. Just as Kurdish forces are advancing on Sinjar in Iraq, the SDF is mounting an offensive near the Syrian city of Hawl 40 miles away. The decision to launch offensives in both Syria and Iraq is a rare, smart move by the U.S.-led coalition, as it forces the Islamic State to fight simultaneously on two fronts. In the past week, both Sinjar and Hawl have been wrested from the Islamic State.

The Syrian coalition has quickly become indispensable to Washington’s war on the Islamic State.

In its current shape, however, the alliance is fraught with mistrust and potentially fatal shortcomings. The group’s greatest strength — its experienced and committed Kurdish leadership — is threatening to become its greatest weakness.

The U.S. focus on the northern Syrian front against the Islamic State began with a renewed appreciation of the Syrian Kurds’ fighting power. According to a senior U.S. official involved in the anti-Islamic State campaign, the battle in Kobani last year marked a turning point in American thinking about how to defeat the jihadi group in Syria. The official said that the Syrian Kurds’ unparalleled commitment to battle the Islamic State prompted the international coalition in January to turn some of its attention to this front and work closely with the Kurds to retake Tal Abyad.

However, the move faced tenacious resistance from Turkey, which worried that a central role for the Kurdish People’s Protection Units (YPG) would empower it internationally.

Ankara considers the YPG connected to the Kurdistan Workers’ Party (PKK), a guerrilla movement that has fought a decades-long war against the Turkish state. U.S. officials spent weeks travelling from Erbil, the capital of Iraqi Kurdistan, to Ankara to ease tensions and address Ankara’s concerns. After the takeover of Tal Abyad, Ankara’s concerns deepened: The U.S. partnership with Syria’s Kurds increasingly seemed to represent a strategic shift toward what the Turkish government viewed as a hostile force that was steadily gaining territory in northern Syria.

Tensions between Washington and Ankara reached a high point in July, when Turkey pushed for an “ISIS-free zone” dominated by Turkey’s allies in Syria that deliberately included territories west of Kobani to the outskirts of Aleppo — an area that Kurds widely regard as part of their historical homeland in northern Syria. The idea represented Turkey’s attempt to put an end to the expansion of the Kurdish influence in the north.

Turkey and the United States therefore reached an understanding, accepted by the Syrian Kurdish leadership, that the YPG will not attempt to expand west into Ankara’s envisioned “ISIS-free zone.” The SDF — which attempts to incorporate both Kurdish and Arab forces fighting the Islamic State in the area, with U.S. assistance — was a consummation of this new understanding.

The Kurdish-led alliance was expected to now focus much of its attention on the northern region of Raqqa, which includes Arab-majority towns and villages that Arab fighters would hold if the Islamic State were expelled. The Kurds, however, prefer to expand their presence in predominantly Kurdish areas rather than fight in areas that would be controlled by Arab fighters. So instead of fighting in Raqqa, as reports first claimed, the new alliance’s attention has turned further east, toward the region of Hasakah.

Military gains in this region — from Shaddadi to Hawl to Malikiyah — will help secure the Kurds’ strongholds along the Iraqi border. Southern Hasakah could potentially provide the Kurds with lucrative resources, including oil fields currently controlled by the Islamic State. Meanwhile, the SDF’s Arab component could also resolve a key dilemma for the Kurds, by providing it with a friendly force to run Arab-majority areas in the area. That would allow the YPG to use its limited resources to attack the Islamic State in the region or deploy fighters elsewhere in the country.

But there is a very real risk that this strategy will not go as planned. If the SDF hopes to break the stalemate in northeastern Syria, it must address a key shortcoming in the alliance: the Arab tribal fighters’ relative weakness compared to their Kurdish allies.

One of the complaints repeatedly heard by Arab fighters within the alliance is that they are poorly armed, as compared to their Kurdish counterparts. They also claim they are deliberately kept weak by the Syrian Kurds, so that they will remain subordinate to the YPG and so that their role is confined to guarding Arab towns.

A senior commander of the Raqqa Revolutionaries’ Brigade, one of the SDF factions, told the authors that uneven American support for the YPG enabled the Kurds to dictate terms to the rest of the factions. The main task of the new alliance “is to protect their areas only because the Kurds can’t cover all the region,” he said. “[The army] has only light weapons so it does not become too powerful.… The American support is what made [the Kurds] above the rest and impose their political goals.”

This reality was exemplified last month, when the Pentagon said that U.S. jets airdropped 50 tons of ammunition to Arab rebel forces in northern Raqqa. However, the Arab factions seemingly could not move the ammunition on their own, and it quickly ended up in Kurdish hands.

There are three reasons the subordinate role for Arab tribal fighters undercuts the alliance’s potential. First, the imbalance will undermine the military capabilities of the coalition to push against the Islamic State in Arab-dominated areas.

Second, the tribal fighters’ status as junior partners in the alliance will increasingly reduce their morale — as happened previously, when many U.S.-trained rebels abandoned the battlefield because they felt the program was aimless and disproportionally focused on counterterrorism. Tribal fighters say that U.S. support for the Kurds indicates it is less committed to tribes in the long term. They fear that nobody would come to their aid if the Islamic State returned to areas from which it had previously been expelled, as happened in Iraq over the years or in the eastern Syrian province of Deir Ezzor last year, when repeated appeals for help went unnoticed by the international community.

“Had it not been for the [international] coalition, ISIS would have reached Qamishli,” said a fighter from the Shammar tribe, which leads the Kurdish-Arab alliance’s al-Sanadid forces. “And the fact is that when ISIS wants, it could reach anywhere.”

Finally, there are widespread fears that as more areas are seized by the Kurdish-led alliance, incidents of ethnic cleansing will increase. Last month, Amnesty International released a report accusing the YPG of committing war crimes, including the forced displacement of Arab civilians and demolition of their houses. “Whenever the YPG enters an area, they displace its Arab residents,” the Shammari fighter said, referring to Arab towns in southern Hasakah. “Fifteen villages were leveled to the ground in Tal Hamees, Tel Brak, and Jazaa.”

Meanwhile, when it comes to the local Arab communities they seek to control, the Arab and tribal factions are widely viewed as lackeys to the YPG. This view was reinforced after the capture of Tal Abyad, when Free Syrian Army factions were marginalized despite initial promises they would help run the city, in addition to the reported incidents of mass displacement detailed by Amnesty. Tal Abyad was a missed opportunity to change the perception about these forces and enable them to mobilize locals and win their support.

At the same time, there are other U.S.-backed groups in eastern Syria — and there’s at least one alternative that skirts the SDF’s inherent ethnic rivalries. The New Syrian Army, a new U.S.-backed militia dedicated to the fight against the Islamic State in the eastern region of Deir Ezzor, consists of fighters who were previously expelled from the area by the Islamic State.

The Kurdish-Arab alliance at the heart of the SDF still has huge potential to reverse the gains made by the Islamic State, whose hold over Syrian territory is much more tenuous than in Iraq. But the YPG should not steer its operations to suit its narrow agenda. Establishing a true balance among the forces that constitute the coalition will boost its military potential and help it better secure both Arab and Kurdish areas held by the Islamic State