Troops fighting ISIS appeared to on the verge of another victory over the self-proclaimed Islamic State Wednesday, as they moved into a city that has served as the main thoroughfare for ISIS foreign fighters and weapons. But the potential seizure of the Syrian city of Manbij by U.S.-backed forces is only likely to set off a new battle for control—this time pitting Arabs against Kurds.

The battle Wednesday reflected a growing problem for the U.S. and its push to train local fighters, even as those forces take territory from ISIS. Who exactly will govern those towns now? Will it be the Kurds who have led the fight against ISIS? Or will it be what some in the Pentagon have privately called the “token Arabs” trained by the U.S. to accompany them?

Two defense officials told The Daily Beast Wednesday they don’t know. They believe the Arabs would be in charge. But even these officials admit that asking the 5,000-or-so newly-trained Arab fighters to control three or more formerly ISIS-controlled areas—and at the same time move into the ISIS capital of Raqqa—would be difficult.

On the other hand, some worry that a Kurdish controlled Manbij could be ethnically cleansed, creating the kind of Sunni disenfranchisement that led to the rise of ISIS. The fall of Manbij into Kurdish hands, however, would give the Kurds a contiguous region in northern Syria. Moreover, a Kurdish controlled Manbij could draw the ire of U.S.-allied Turkey, which rejects a Kurdish controlled region on its border.

The question “what happens after ISIS?” looms increasingly over the U.S.-led effort. Indeed, defense officials said how the governance question is answered in Manbij could foreshadow the strategy for Raqqa, ISIS’s capital. Local U.S.-backed forces, accompanied by U.S. forces, have moved within 18 miles of the city in the last week. Over the Memorial Day weekend, one U.S. service member was injured while supporting the local fighters.

“ISIS is losing. The U.S needs to make sure what emerges next is not an utter wreck that will allow this brutal organization or something like it to move back in,” Daveed Gartenstein-Ross, a senior fellow at the Washington, D.C.-based Foundation for Defense of Democracies, explained to The Daily Beast.

U.S. officials could once defer answering about what happens after ISIS, saying their focus was to end the terror group’s brutality. But the militants’ steady stream of defeats is making such questions unavoidable. The U.S.-led airstrike campaign has increasingly assaulted ISIS logistical operations, forcing the terror group to retreat from territory it once controlled. Such losses have made it harder for ISIS to move weapons, food, and fighters around the self-proclaimed caliphate and appear to have weakened the group’s ability to expand its state across the Middle East and Africa.

In recent months, ISIS has lost control of Ash Shaddadi, a key logistical city east of Raqqa, several town north of the ISIS capital. And now it appears that the Iraqi city of Fallujah, the last bastion of ISIS control near Baghdad, is in danger of falling out of ISIS hands.

At the organization’s peak nearly two years ago, thousands of ISIS fighters entered through Manbij each month and moved throughout Syria and Iraq. U.S. military estimates now put that number at closer to 500 a month, as it appears ISIS has instead spread its foreign fighters across the region to places like Libya and Egypt. The terror group has even asked recruits to stay in Europe and attack from there.

The Kurds have set the tone for the war against ISIS, saying for months they wanted to take places like Manbij before approaching Raqqa, the ISIS capital.

On Wednesday, more than 2,000 fighters entered Manbij, which has served as a key thoroughfare between Turkey and Syria. According to one report, the fighters have taken 20 villages and are fewer than 10 miles from the city center.

The forces have received American help, which signaled that the coalition was willing to risk aggravating Kurdish/Arab tensions and the Turks in exchange for a liberated Manbij. Since May 27, the U.S.-led coalition has conducted 36 airstrikes around Manbij, including 18 in the last day, according to Defense Department (DoD) statistics. And according to the Syrian Observatory for Human Rights, the latest strikes have killed at least 15 civilians.

The fighters charging toward Manbij are members of the Syrian Democratic Forces (SDF), a Kurdish-dominated, U.S.-backed force recently bolstered by 5,000 U.S.-trained Arab fighters. In all, there are roughly 25,000 SDF, according to DoD figures.

When asked who would control Manbij if ISIS fell, one defense official demurred, saying instead, “Right now, the SDF are closing the flanks.”

The military plan for Manbij is for Arab fighters to lead the charge, backed by Kurds, and then hold the city. But just how that would play out remains unclear, defense officials conceded. What happens if the Arab fighters need help from the Kurds? Will they receive it? And if so, will the Kurds demand more in concessions for the city?

The potential fall of Manbij with the help of the Kurds, along with the fall of areas around Raqqa, suggest that the U.S. is willing to risk creating potential new tensions to rid the northern city of ISIS.

“All signs point to the de facto establishment of a contiguous Kurdish-controlled territory in northern Syria, which the United States will have had a hand in creating,” Aaron Stein, the senior resident fellow for Turkey with the Rafik Hariri Center for the Middle East, concluded in a February assessment.

The retaking of the Manbij pocket—and particularly the town of al-Bab, where ISIS’s foreign intelligence apparatus is headquartered — would represent a hammer blow to ISIS’s operations in northern Syria. For that reason, the terror group has continued to demonstrate a willingness to hang onto its holdings there. In recent days, its own progress on the battlefield has come as a result of exactly the sort of sectarian conflict between Arab and Kurd that CENTCOM is seeking to avoid.

The Aleppo town of Marea, for instance, is now completely surrounded by both ISIS and the U.S.-backed Kurdish militias—even though the 400 Syrian rebels of Mu’tasim Brigade are also assets of the U.S. military, as the Pentagon has confirmed to The Daily Beast.

According to Mustafa Sejry, the head of the political office of the Mu’tasim Brigade, ISIS has dispatched 1,000 jihadists to try and invade Marea, roughly the same force contingent ISIS currently has deployed against 20,000 pro-Iraqi ground troops in the city of Fallujah.

“ISIS cut off the road between Azaz and Marea six days ago,” Sejry said, referring to a crucial Syrian-Turkish border town that ISIS has also made a major play to recapture. “After that, Marea became completely besieged by ISIS. We’re engaged in operations to break the siege. We were making progress on the second day, but we were surprised that the coalition struck one of our groups, killing 10 of our fighters and injuring 12.”

Sejry shared with The Daily Beast photographs purporting to show the damaged caused by these alleged U.S. airstrikes. These images could not be independently confirmed, however.

“Since then, we’ve been asking the Americans to airdrop support for us,” Sejry said.

Fifty fighters from the Mu’tasim Brigade are graduates of the Pentagon’s “train and equip” program for recruiting Sunni Arab and Turkmen counterterrorists from the ranks of the Free Syrian Army to take on ISIS. These men were trained at a U.S.-administered military base in Turkey.

From its inception, the train-and-equip program has been plagued by setbacks and failures, with the first class of graduates having been kidnapped by Jabhat al-Nusra, the Syrian al-Qaeda franchise, and members of the second class having sold American supplies to that same franchise, as The Daily Beast was the first to report.

But Sejry insists that the Mu’tasim Brigade has been an exception to that ignoble trend. “There is no incident of our weaponry being given or sold to any other group, not even other FSA groups,” he said. “The U.S. government has always been very happy and enthusiastic to work with us.”

For months, the Mu’tasim Brigade had received “regular shipments" from the United States of mortar shells, M16 and M2 rifle ammunition, vehicles, clothes and belts, he added. “But since the last shipment we have not received any supplies. We’ve given the U.S. coordinates where they can safely drop support for us.”

So why did the shipments stop? The reason given to the Mu’tasim sounded implausible. “The Americans told us that they feared that if they dropped supplies to us, these might fall into the hands of the PYD,” Sejry said, using the acronym of the Kurdish political party that controls the Syrian Democratic Forces. “And they said they don't want that to happen. That’s unconvincing to us because we’re fully aware of U.S. support to the PYD.” The SDF, Sejry added, have allowed 200 families (mainly women, children and the elderly) to pass through its territory out of Marea, but this was done in exchange for control of the small town of Sheikh Aisa in Aleppo. "They are fighting us psychologically," Sejry said.

Navy Commander Kyle Raines, a CENTCOM spokesman, told The Daily Beast on Tuesday, that, so far as he is aware, U.S. supply runs to anti-ISIS forces have not stopped.

“We’ve been resupplying the vetted Syrian coalition and other folks all along,” Raines said. “I’m not in a position to contradict what [Sejry] is saying but we have continued to resupply people.” Possibly delays, Raines offered, could owe to battlefield conditions that make cargo runs or airdrops impossible.

But how does the U.S. reconcile two of its own ground proxies leery of and at odds with each other while ISIS simultaneously targets one of them? Raines declined to comment.

On Wednesday evening, after inquiries made by The Daily Beast to CENTCOM, the Mu’tasim Brigade received confirmation from the Pentagon that it would be resupplied imminently.UPDATE: Shortly after this article was published, Mustafa Sejry sent The Daily Beast photographic confirmation that the Mu'tasim Brigade had received a new air drop of military equipment.