Although it has been overshadowed by the dramatic developments in Aleppo, this past weekend the U.S.-backed Syrian Democratic Forces broke the resistance of the self-declared Islamic State in nearby Manbij. Pockets of jihadi resistance remain inside the city, but the battle seems to be essentially over. As it becomes possible to take stock of the situation in Manbij and a new administration moves in, the city will be seen as an important bellwether in the war against the Islamic State.

A rural town with a pre-war population of more than 100,000 people, Manbij has been a stronghold of the Islamic State since 2014. Located in the Syrian-Turkish border region east of Aleppo, it served as an important hub for trade, smuggling, and the movement of foreign fighters. Capturing it was an essential step in breaking the Islamic State’s hold on eastern and northern Syria. Despite this, Manbij was long left under uncontested jihadist control. All rival forces in the region—whether those loyal to President Bashar al-Assad’s government, some faction of the Sunni Arab opposition, or the Syrian Kurds—were busy fighting each other instead of the Islamic State.

The current push on the city began on May 31, when the Syrian Democratic Forces expanded from a small foothold west of the October Dam on the Euphrates to envelop Manbij. With heavy American and European air support, the group could surely have taken the city much more rapidly by simply leveling it block by block. But instead, the attackers decided to lay siege to the city and gradually fight their way to the center. However, the jihadists fought them block by block with snipers and booby-trapped buildings, which led to a protracted battle with terrible consequences for the city and its population. In one incident on July 19, scores of civilians were killed in U.S. air strikes on Manbij and a nearby village, with some sources putting the number of dead at 117.

Why Manbij Matters

The details of the Manbij siege are important for what they tell us about the war on the Islamic State and the impact of American air power, but also by demonstrating the evolving nature of the Syrian Democratic Forces. This secular coalition of Kurds, Arabs, and Syriacs, created in October 2015, is the most important ally of the United States in Syria and the most effective anti-jihadist force in the country.

However, it is also in some ways a problematic ally for Washington. Its membership is overwhelmingly dominated by a powerful Kurdish militia known as the People’s Protection Units, or YPG. The YPG and the closely affiliated Democratic Union Party (PYD) are widely regarded as front organizations for the Kurdistan Workers’ Party (PKK), a Kurdish group fighting the Turkish government that has been labeled a terrorist organization by both Ankara and Washington. The U.S. government has played down the YPG’s connection to the PKK in order to avoid legal and political challenges to its actions in Syria, but Turkey remains bitterly opposed to the American-Kurdish pact in Syria.

More importantly, northern Syria is a tinderbox of ethnic tensions between Arabs and Kurds, as well as between local Arab tribes. In many places, preexisting hostilities have fed into the wider Syrian war, with villagers backing the “Arab” Islamic State or the “Kurdish” Syrian Democratic Forces along ethnic lines, or to get the upper hand over local rivals. With all sides enjoying some level of grassroots support, civilian communities are often targeted as a practical way of putting pressure on the enemy. Arab groups have thus repeatedly shelled and attacked Kurdish civilians, while Kurdish forces have committed war crimes of their own against Arabs.

These ethnic and political cracks run straight down the middle of U.S. strategy. Washington has struggled to keep the peace among its Syrian allies, who tend to hate each other more than they hate the Islamic State. In July, the Arab-dominated opposition in exile, which is based in Turkey and promoted by the United States as an alternative to Assad, was so angered by the death of civilians in Manbij that it demanded a suspension of the U.S. air war against the Islamic State in Syria. Similarly, Kurds from the Syrian Democratic Forces in the Aleppo area have at times worked with Assad’s government and Russia to undermine U.S.-backed Arab rebels.

Moving Arab Factions to the Front

The United States needs to tread carefully to avoid inflaming ethnic tensions, alienating its own allies, and driving Arab rebels into the arms of the Islamic State. But so far, U.S. efforts to recruit Sunni Arab fighters against the Islamic State have met with very little success, leaving Washington ever more dependent on the Kurdish-dominated Syrian Democratic Forces. To find a way out of this dilemma, the Pentagon has sought to channel extra support to certain small Arab factions inside the Syrian Democratic Forces, hoping that this will at some point enable the coalition to transcend the ethnic divide. While these groups started out weak and completely dependent on Kurdish and American support, the idea has been to gradually empower them and bring them to the forefront as the war moves deeper into Arab-majority territory.

Manbij was supposed to be the testing ground for this strategy, as the first significant city taken by Arab-led components of the Syrian Democratic Forces. According to one U.S. military official who spoke to Reuters at the start of the offensive, YPG troops would assist their Arab partners but then leave the territory, in order to “have Syrian Arabs occupying traditional Syrian Arab land.” Turkish President Recep Tayyip Erdogan said in early June that only about 450 members of the YPG were involved the Manbij offensive, compared to some 2,500 Arab fighters. Another U.S. official gave the slightly lower figure of 2,000 Arab fighters, but agreed that Kurdish forces were mostly participating in a supporting role.

Whether these numbers are accurate is impossible to say. Though Arab groups do seem to represent a major part of the force in Manbij, many doubt that they could operate independently of their Kurdish allies or rule the city without YPG support and consent.

What Works in Manbij May Work in Raqqa

If the Syrian Democratic Forces can wrest Manbij from Islamic State control without leaving it in ruins, and then proceed to establish a solid relationship with the local Arab population, the city may serve as a model for future campaigns. Ultimately, the United States wants the Syrian Democratic Forces to take the war to Raqqa, the Islamic State’s administrative capital in Syria, which is considerably larger.

The results of the Manbij campaign will be carefully scrutinized and analyzed to judge the Syrian Democratic Forces’ ability to move on Raqqa. As the smoke clears, it remains to be seen how much of the city is still standing and whether aid and reconstruction efforts can revive civilian life. Just as important, the new administration put in place by the Syrian Democratic Forces will need to stave off chaos and repress Islamic State subversion, while securing enough buy-in from local Arab populations to avoid a relapse into ethnic conflict.

There are tremendous obstacles ahead, but if the strategy works as intended, then for the first time in two years the idea of an offensive against Raqqa will begin to sound feasible.