The U.S., the Islamic State militant group (ISIS) and fighters supportive of Syrian President Bashar al-Assad have all recently engaged in battles with one another in the eastern province of Deir Ezzor, but recent developments have pulled both pro-Syrian government fighters and U.S.-backed Kurds away from this battle and united them against a new, common enemy.

The Syrian Democratic Forces, a secular and largely Kurdish group established by the Pentagon in October 2015, has been the leading ally of the U.S. in Syria since relations with the mostly Sunni Muslim Arab opposition trying to overthrow Assad since 2011 fell out. As Washington's priorities shifted from regime change to defeating ISIS, the Syrian Democratic Forces served as the vanguard to U.S. ground efforts to fight the jihadis, which have been largely defeated by both pro-Syrian government and U.S.-led campaigns as of late last year.

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Turkey—an unsteady U.S. ally in the Middle East and fellow member of the NATO Western military alliance—considered a number of Kurdish militant groups operating within the Syrian Democratic Forces to be terrorist organizations on par with ISIS. In January, Turkey launched a joint offensive with the former CIA-backed Free Syrian Army to dismantle Kurdish control over the northwestern enclave of Afrin. As result, thousands of Kurdish fighters have reportedly left their positions in the east and the U.S. has struggled to mediate between two of its allies in Syria.

"The Coalition maintains that any partner effort in Syria not directed at defeating Daesh is not helpful," a spokesman for the U.S.-led coalition said in a statement sent Thursday to Newsweek, using the Arabic-language acronym for ISIS.

"We are aware of the departure of some SDF forces from the Middle Euphrates River Valley and continue to point out the potential costs of any distraction from the Defeat-ISIS fight," he added.

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The coalition also said that Syrian Democratic Forces commander General Haval Mazloum was now arranging his fighters "to meet the demands of a complex situation and we acknowledge his leadership decisions under our long-standing approach of working by, with and through our partners to achieve the lasting defeat of Daesh and ensure Syrian answers to Syrian challenges." It stated that "no other force could have achieved such a rapid defeat of Daesh, followed by the flourishing of local, representative government."

The dominant faction of the Syrian Democratic Forces, however, is the People's Protection Units (YPG), one of the Kurdish militias that Turkey has linked to a violent, decades-long insurgency by the Kurdistan Workers' Party (PKK). Awkward clashes have frequently broken out between the Pentagon-backed Syrian Democratic Forces and the Free Syrian Army, the premier recipient of the CIA's doomed "rebel train and equip program" intended to oust Assad, who the West has accused of human rights abuses.

These tensions grew as ISIS lost out to the pro-Syrian government and U.S.-led campaigns in the eastern province of Deir Ezzor. Addressing U.S. support for the YPG, Turkish Foreign Minister Mevlut Cavusoglu warned in January that "the fight against terrorism cannot be won by siding with one terrorist organization against another" in a Newsweek op-ed.

The crisis erupted shortly after the U.S.-led coalition suggested days later that it was forming a 30,000-strong border security force to be maintained by the YPG-dominated Syrian Democratic Forces. Turkey vowed to crush the "terror army" and rallied its Free Syrian Army allies to launch the so-called "Operation Olive Branch" that, despite fierce resistance, has made slow gains in Afrin over the past month.

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The Pentagon pushed back, saying "the Afrin operations are impeding the task to eliminate ISIS" and refusing to remove U.S. troops deployed in the Kurd-held city of Manbij. Ultimately, the U.S. acknowledged Turkish concerns about potential Kurdish infiltration of the border and has not moved to halt the Turkish offensive. Russia has provided support to Kurdish forces in the past, but saw Turkey as part of its own efforts to solve the seven-year civil war in Syria and also declined to intervene directly in Afrin.

As neither the U.S. nor Russia would risk their relationship with Turkey by attempting to stop the attack and Syrian Kurds watched U.S. abandonment of Kurds in Iraq, Syrian Kurdish leadership reached out directly to Assad, with whom they had a complicated relationship, but one less hostile than their connection to Turkey and its Free Syrian Army allies as of late.

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"Who's going to fight and die for us when we're not a reliable ally?" David L. Phillips, director of the Program on Peace-building and Rights at Columbia University’s Institute for the Study of Human Rights, told Newsweek.

"They turned to Assad, who's no friend of the Kurds, but we gave them no choice," Phillips added. "Make no mistake, the Free Syrian Army is a jihadist force. They mutilate the bodies of Kurdish fighters. The idea that we'd see the Free Syrian Army as anything but a terror army is ludicrous."

Describing the relationship between the YPG and the Syrian government as a "transactional reliance" that could potentially help Kurds achieve greater autonomy in a post-war Syrian constitution, Phillips said the U.S. had effectively chosen the interests of Turkey and its allies over those of the Kurds.

The YPG has fought both alongside and against Syrian troops throughout the seven-year civil war, but the Kurdish community's hopes for autonomy and their U.S. backing has kept them mostly apart from Damascus. Like Turkey, Syria also labeled the YPG a terrorist organization and Assad's father and predecessor actually won Turkey over by extraditing PKK leader Abdullah Öcalan in 1998. The militant leader's face has been frequently featured on the flags and uniforms of the YPG and its allies.

As the war progressed, the younger Assad has since viewed Turkey as the greater enemy, due to its support for Syrian insurgents and incursion into Syrian territory. After a deal was reportedly reached last month between the Syrian government and Kurdish forces, pro-Assad convoys arrived at the frontlines of Afrin.

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A Russian military intervention in support of Assad in September 2015, a month before the Syrian Democratic Forces were officiated, helped the Baathist Syrian leader once again emerge the primary political actor in Syria. The momentum behind the 2011 uprising dissipated with infighting and growing jihadi influence among opposition groups. After devoting a substantial amount of resources to various insurgents, the U.S. too cut assistance to those insisting on fighting Assad instead of ISIS.

As the U.S.'s new top partner, the Syrian Democratic Forces stormed the de facto ISIS capital of Raqqa in October. The following month, the Syrian military—supported by Russia and various allied militias, many of which were local and international Shiite Muslim forces backed by Iran—retook the eastern jihadi stronghold of Deir Ezzor, where some Syrian troops had been trapped under siege for more than three years. Reports of violence arose on lines of control shared by the Syrian Democratic Forces and pro-Syrian government forces, but they remained minor. Since the Afrin front opened on the other side of Syria, however, the dynamics have changed.

The Syrian Democratic Forces included a large number of Arab fighters in addition to Kurds and other ethnic minorities, but Kurds have always filled out its leadership positions. The Kurdish flight northwest threatened to leave the largely Arab component of the Syrian Democratic Forces active in Deir Ezzor in disarray.

The result could spell disaster as fresh tensions over nearby oil fields have pitted the mostly Arab factions of the Syrian Democratic Forces against pro-Syrian government fighters. U.S. strikes on pro-Syrian government forces earlier this month left up to 100 dead, including Russian citizens. The U.S. said these clashes were provoked by pro-Assad forces trying to take back natural resources previously seized by ISIS, but Russia and Syria have said the U.S. was the aggressor and was protecting the jihadi group.

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"In a way, ISIS belied important geopolitical differences that are now becoming clearer. ISIS was a great unifier in the sense that virtually all international actors opposed the group and were therefore prepared to set aside differences to work towards this common goal," Max Abrahms, assistant professor at Northeastern University and expert on international terrorism, told Newsweek.

"As ISIS has waned, though, we are seeing strategic links fray and diverges to grow sharper between the U.S. and Turkey, Russia and the U.S., the U.S. and Iran, and the costs of Washington siding with the Kurds is getting tested," he added.

Abrahms also pointed out recent reports indicating that Turkish airstrikes have killed pro-Syrian government forces, escalating what has grown into a war between states in which the U.S. has no clear position. With Kurdish support dwindling and with the Syrian government's outreach campaign increasingly winning over the mostly Arab tribal areas of Deir Ezzor, even U.S. military leaders have cast doubt on Washington's ability to strike the final blow against ISIS and maintain a serious stake in Syria's future.

In his testimonial to the House Armed Services Committee on Tuesday, U.S. Central Command head General Joseph L. Votel echoed the coalition's words in calling the Syrian Democratic Forces "the most effective force on the ground in Syria against ISIS," as The New York Times reported.

"We need them to finish this—to finish this fight."

This article was first written by Newsweek

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