Here's why U.S. allies are fighting each other in Syria

The United States’ attempts to bring together bitter enemies to tackle the Islamic State in Syria are unraveling, as Washington’s key partners turn their guns on each other.

Over the past few days, Syrian rebels backed by the Turkish military have clashed with Kurdish fighters inside Syria. That's thrown into chaos an anti-Islamic State alliance that includes all of the above, and is led by U.S. forces.

Turkey sent tanks, artillery and planes into Syria last week in support of the rebels in order to drive the Islamic State from the border town of Jarablus. The U.S. endorsed the mission and provided drones for surveillance and aerial support.

The operation appears to be going in a different direction from the one the U.S. had hoped. Turkish-backed forces are mostly focusing on attacking a Kurdish-dominated alliance with Arab fighters called the Syrian Democratic Forces, south of the town.

An intervention the U.S. initially welcomed now seems like it could benefit the Islamic State, in the short term, by setting the extremist group’s enemies against each other.

Turkish airstrikes have blasted Kurdish-held areas east and west of the Euphrates River, and helped the Turkish-allied Syrian rebels advance.

The Syrian Observatory for Human Rights, a monitoring group based in Britain, said at least 40 civilians were killed in Turkish shelling and airstrikes on Sunday. Officials in Turkey’s capital Ankara rejected the claim.

Turkey, meanwhile, said it killed 25 Kurdish “terrorists” in strikes on Sunday. A day earlier, a Turkish soldier was killed in what Turkey said was a rocket attack by the Syrian Kurdish People’s Protection Units, or YPG.

Militias aligned with the Syrian Democratic Forces said Kurdish fighters had left the area Turkey was targeting. Ankara denies the claim.

The fighting comes when the U.S. and Russia say they’re working toward an end to Syria’s more than five years of civil war. But Russia — together with Iran — backs Syrian President Bashar Assad. The U.S. and allies have supported various forces opposing him.

In the battle against the Islamic State, however, the U.S. has cobbled together an unlikely array of proxies and partners to tackle the group.

U.S. fighter jets take off from an airbase in Turkey to bomb Islamic State strongholds in Syria and Iraq — and to provide air support for the very Kurdish fighters that Turkey is now targeting. Washington and Moscow are talking about cooperating and sharing intelligence to better target the Islamic State.

But even when faced with an enemy as loathed as the Islamic State, not everyone can get along.

Although they share an enemy, Syria’s Kurds and Ankara dislike each other almost as much. Turkey claims that the YPG in Syria is part of the Kurdistan Workers’ Party, or PKK, which is considered a terrorist organization by Turkey, the European Union and the U.S.

Turkey is fighting a bitter conflict against the PKK in Turkish cities. The Turkish authorities worry that gains made by Syrian Kurds toward autonomy in Syria would rile Kurdish separatists inside Turkey even more. Its intervention in Jarablus aimed at preventing Kurdish forces from grabbing sizable land along the border.

Syrian Kurds see Turkey as an abuser of Kurdish rights, and a barrier to their aspirations of greater autonomy.

The U.S. has tried to balance Kurdish aspirations in Syria with Turkey’s concerns, apparently unsuccessfully.

Months ago, Turkey warned that the Euphrates was a red-line it would not allow Syrian Kurdish fighters to cross. When the YPG did cross that line, it was with U.S. support — when the Syrian Democratic Forces captured the city of Manbij from the Islamic State earlier in August.

Just days after a U.S. spokesman for the anti-Islamic State coalition celebrated the Kurdish-led capture of Manbij on Twitter, calling the YPG-dominated Syrian Democratic Forces a “stalwart ally,” those same forces were being attacked by Turkey.

The U.S. demand for the YPG to withdraw east of the Euphrates was seen by some as a slap in the face for a group that had spilled blood capturing Manbij.

The YPG has since said it would withdraw, while leaving an allied unit behind in Manbij.

Turkey has vowed to keep up the pressure until the Syrian Kurdish fighters pull out completely from the area.

“The YPG ... needs to cross east of the Euphrates as soon as possible. So long as they don’t, they will be a target,” Turkey’s Foreign Minister Mevlut Cavusoglu said on Monday.

He accused the group of “ethnic cleansing,” by taking control of traditionally Arab areas of northern Syria.

Pentagon spokesman Peter Cook said Monday the U.S. was working to stop the fighting among its allies along the border. He added that the Turkey-U.S. demand for the YPG to retreat east of the Euphrates had “largely occurred.”

If the U.S. can convince the YPG to fully withdraw to the east of the Euphrates, there is a chance that the U.S. may have gained a more committed player in the fight against the Islamic State in the form of Turkey. If not, a drawn-out conflict between America’s allies could allow the Islamic State time to regroup right when it’s being squeezed on all sides.

This story originally appeared on GlobalPost and PRI.org. Its content was created separately to USA TODAY.

MORE FROM GLOBALPOST:

A Puerto Rican enclave in Brooklyn confronts the realities of rising prices and gentrification

Singapore is dealing with an outbreak of locally transmitted Zika infections