Men walk near a damaged building in Aleppo's rebel held al-Fardous district, Syria May 26, 2016. (Abdalrhman Ismail/Reuters)

On May 26 the Islamic State (ISIS or ISIL) launched a successful westward offensive in Syria’s northern Aleppo countryside. ISIS effectively encircled thousands of insurgent fighters in the town of Marea and is now less than 2 miles from the critical opposition-held city of Azaz. Although ISIS only seized six villages and may yet lose them, that does not reveal the full extent of the danger this attack poses. Aleppo province is where multiple axes of the Syrian war intersect: the regime-opposition conflict; Kurdish-insurgent competition; Russia’s military role; US policy toward the rebellion (and therefore the regime); Turkey’s critical geopolitical interests; and of course, the multi-front war on ISIS. The real significance of military developments in Aleppo usually goes beyond territorial changes. For the insurgency and its Turkish patron in particular, the margin for error in a critical part of Syria is now vanishingly thin. The implications for the US war on ISIS are alarming.

In recent weeks, US and Turkish-backed rebel groups have repeatedly fought ISIS, in an attempt to drive the group out of Aleppo province. The insurgents captured territory but were unable to hold it against ISIS counterattacks. Over the weeks, Russian-backed regime operations, ISIS attacks, and Kurdish PYD offensives further squeezed the insurgency in Aleppo, eventually confining it to the narrow “Azaz corridor” and a single critical but vulnerable supply line running through it from Turkey. Surrounded by hostile PYD forces to the west, ISIS to the east, and the regime to the south, the insurgents in the corridor were already in danger before the recent ISIS offensive. With all these actors positioned to make land grabs in the area, and rebels exhausted by months of fighting, the corridor is now on the verge of collapse.

A number of scenarios could destroy the corridor and, with it, the rebel presence in Aleppo province (with the exception of soon-to-be-encircled Aleppo city). First, the PYD could move on opposition territory, under the pretext of defending it from ISIS. The PYD could grant these insurgents safe passage through Kurdish-dominated territory instead of attacking them, but that seems unlikely given mutual hostility. ISIS may also continue its westward offensive in a race with the PYD for the corridor, and would likely execute any rebel prisoners. Regime and Russian air strikes on the insurgents would facilitate either ISIS or PYD movement. That would be consistent with a regime policy of prioritizing the war on the insurgency.

If the Azaz corridor collapses, the most obvious loser (apart from the opposition of course) will be Turkey. It would no longer have proxy capability in northern Syria’s most strategic province, in a war where critical national security interests are at stake. These interests include, but are not limited to, containing expansion by the PYD – a group closely linked to Turkey’s historical enemy the PKK – in part through Turkey’s rebel allies in Aleppo. With Kurdish-led forces, ISIS, the Al Qaeda affiliate Jabhat al-Nusra, and the regime controlling most of northern Syria, Turkey would be nearly out of allies in Syria.

US policy in Syria is more narrowly defined and largely focused on defeating ISIS, and the collapse of the Azaz corridor would significantly complicate that fight. Presently, the United States and Turkey have a two-pronged approach to fighting ISIS. East of the Euphrates river, the United States supports a largely-Kurdish military effort against ISIS, the PYD-led Syrian Democratic Forces (SDF). This ethnic imbalance has strained Kurdish-Arab relations in areas liberated from the group, amid reports of ethnic cleansing of Arabs by the PYD. The United States however, unwilling to get more deeply involved in the Syrian conflict, calculates that this is a risk worth taking. West of the Euphrates however, and under strong Turkish insistence, there is an Arab opposition-led anti-ISIS effort. The so-called Hiwar Kilis Operations Room includes Turkish-backed Islamists and US-backed nationalist insurgents and has enjoyed Turkish artillery and US aerial support.

The United States needs local Arab partners against ISIS, such as the Hiwar Kilis Operations Room. It also needs Turkish cooperation against ISIS which is essentially contingent on keeping Kurdish forces east of the Euphrates. Without a significant Arab force in northern Aleppo, and in the absence of a broader more ambitious policy to end the civil war, the United States will be tempted if not compelled to rely completely on the PYD and allow it to expand westward into Arab territory. That would be catastrophic for US-Turkish relations and the related anti-ISIS effort. An exclusively Kurdish war on ISIS would deepen Sunni extremism among an aggrieved Arab population, deprive the United States and its allies of Arab partners in a crucial geography, and eliminate the moderate opposition in northern Syria.

US military planners recognize the tension between beating ISIS and the limited local tools available to them. Much of the Arab population in Raqqa and Deir al Zour is disorganized, disoriented and, understandably, fearful of ISIS and resentful of the PYD. The United States is trying to address that by augmenting the Arab component of the PYD-led SDF. This same logic ought to apply doubly in Aleppo, where local fighters with experience and commitment have fought ISIS for years. At the moment, overwhelming Turkish artillery and US airpower are needed to help them push ISIS back, and expand the eastern boundaries of the Azaz corridor. In the longer run, if the corridor survives, this crisis only highlights how much the United States needs a well-armed, well-trained Arab insurgent partner in Aleppo.

Faysal Itani is a Resident Senior Fellow with the Atlantic Council’s Rafik Hariri Center for the Middle East.