Will U.S. Take a Side in War Between Two of America's Allies in Syria?

A banner bears the Turkish flag and a picture of Turkish President Recep Tayyip Erdoğan, and Syrians wave rebel flags, during a demonstration in the rebel-held town of Azaz, Syria, on January 19, in support of a joint rebel-and-Turkish military operation against U.S.-backed Syrian Kurdish forces. NAZEER AL-KHATIB/AFP/Getty Images

The U.S.'s primary ally in Syria has come under attack from another partner, Turkey, putting the Pentagon in a difficult position as a new front opened violently between two friendly forces.

Turkish forces began an assault Friday on the northern enclave of Afrin, controlled by the Syrian Democratic Forces, a U.S.-backed, mostly Kurdish alliance that includes Arabs and ethnic minorities. The operation came amid conflicting messages from the U.S., which initially suggested it was building a 30,000-strong force along the Turkish border, angering Ankara even after Washington backpedaled. Turkey considers Kurdish militias such as the People's Protection Units (YPG) terrorist organizations and has long threatened to defy the U.S. and attack them.

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"The operation has actually, de facto, started with cross-border shelling," Turkish Defense Minister Nurettin Canikli said Friday, according to Reuters, adding that no Turkish soldiers had yet entered Afrin.

"All terror networks and elements in northern Syria will be eliminated. There is no other way," Canikli said. "The operation in central Afrin may last a long time, but the terrorist organization will swiftly come undone there."

A banner bears the Turkish flag and a picture of Turkish President Recep Tayyip Erdoğan, and Syrians wave rebel flags, during a demonstration in the rebel-held town of Azaz, Syria, on January 19, in support of a joint rebel-and-Turkish military operation against U.S.-backed Syrian Kurdish forces. NAZEER AL-KHATIB/AFP/Getty Images

Those "terror networks," especially the YPG, have dominated the U.S.'s effort to battle the Islamic State militant group (ISIS) in Syria. The U.S. established the Syrian Democratic Forces in October 2015, a year after the U.S. and a coalition of countries began bombing ISIS, and a month after Russia entered the battle in support of Syrian President Bashar al-Assad. As battlefield losses, infighting and growing jihadi influence led the U.S. to cut back support to rebel groups trying to oust Assad, the Pentagon focused on the Kurdish front.

The U.S.-led offensive made significant gains across northern Syria, taking ISIS's de facto capital of Raqqa in October. Turkey, which remained supportive of the embattled Syrian opposition, has been deeply critical of the rise in Kurdish militant activity near its southern border, alleging links between the YPG and the banned Kurdistan Workers' Party (PKK) that has waged a decades-long insurgency at home.

As ISIS remained on the verge of total defeat, the U.S continued to focus on bolstering its Kurdish partners as a bulwark to the growing Iranian influence that came with nationwide Syrian military victories. Analysts said the U.S. may have to rethink this strategy, however, if it wanted to remain aligned with Turkey, a fellow member of Western military alliance NATO.

"U.S. and Turkish policies have become a zero-sum game. Turkey is at war with the U.S. local partner, and therefore, indirectly, at war with the United States. Disputes over the Syrian Kurdish YPG is a key obstacle precluding alignment between Turkey and the U.S. on other shared goals, such as the departure of Assad. The U.S. underestimates the scale of the current rift and the requirements to solve it," Elizabeth Teoman, a Turkey analyst at the Institute for the Study of War, told Newsweek.

"The U.S. continues to base policy on the myth that the Syrian Kurdish YPG is not linked to the Kurdistan Workers' Party (PKK), and therefore to the ongoing domestic insurgency in Turkey. Turkey will continue to escalate until U.S. policy adjusts to reality," Teoman continued.

Syrian Kurds march during a protest in support of Afrin in the northern Syrian town of Jawadiyah on January 18. Syria's Kurdish minority considers northern Syria part of their ancestral homeland and has pushed for greater autonomy, frustrating the Syrian government and infuriating Turkey, which has its own restive Kurdish population. DELIL SOULEIMAN/AFP/Getty Images

While Turkey has repeatedly condemned Assad and called for his downfall, the Syrian leader's victories, especially the retaking of Aleppo in December 2016, have compelled Turkey to work with Russia and Iran to find a diplomatic solution. Those talks, along with Ankara's procurement of an advanced Russian missile defense system, have further strained the already troubled relationship between Turkey and the U.S. in Syria.

Last week, Turkey summoned the envoys of Iran and Russia over Syrian military incursions into the last rebel-held province of Idlib as well as the U.S. charge d'affaires over the Pentagon's backing of Kurdish fighters. Expressing grievances against all sides of the conflict, Turkey chose only to take action against the U.S.-backed contingent, and reportedly coordinated Russia and Iran to do so.

The relationship between the U.S. and Turkey may have hit its low point, but it does not necessarily mean a new war is on the horizon. A new conflict could emerge, however, if Turkey or the YPG escalated too far and the U.S. was forced to take a side.

"The U.S. likely will not respond militarily to a Turkish attack on Afrin but will attempt to deter a Turkish attack on Manbij, which Erdoğan also threatened to take by force. The primary concern for U.S. forces in Syria, moving forward, will be to contain the follow-on escalation between Turkey and the Syrian Kurdish YPG, which has threatened a wide-scale retaliation if Turkey attacks," Jennifer Cafarella, the Institute for the Study of War's senior intelligence planner, told Newsweek.

"The U.S. will focus on de-escalation and will likely attempt to set red lines for a YPG counter-escalation to avoid a widening war that expands into Turkish territory. YPG attacks inside of Turkey could force the U.S. to reconsider the nature and extent of U.S. support to YPG forces in Syria," Cafarella elaborated.

A Syrian government-forces member walks in the village of Tal al Daman, in Syria's Aleppo province, on January 18. GEORGE OURFALIAN/AFP/Getty Images

As the YPG shared reports Friday of an attempted Turkish military incursion into Afrin, the U.S. remained on the sidelines. Fears of U.S. abandonment have long undermined Kurdish trust in Washington, and the Syrian Democratic Forces have increasingly engaged with Russia in past months.

Syrian Kurds also have entered into negotiations with the Syrian government, potentially over swapping Kurd-controlled oil fields in the eastern province of Deir Ezzor—where both groups fought in separate campaigns that have mostly defeated ISIS—in exchange for greater political autonomy in northern Syria.

Russia also stood in the middle of the conflict between Turkey and Syrian Kurds. Turkish reports earlier this week suggested that Russian observers vacated Afrin in anticipation of the upcoming offensive, but Russian Foreign Minister Sergey Lavrov denied this on Friday, according to Russia's official RIA Novosti outlet.

While Lavrov joined Turkey, Iran and Syria in condemning the alleged U.S. border security plan that initiated the current conflict, Moscow has worked directly with the YPG in their common fight against ISIS, and sought to include Kurdish groups in the Syrian National Dialogue Congress set to be held in Sochi, Russia, in hopes of reconciling warring factions in Syria.