The U.S.-Turkey alliance is in a downward spiral

Relations between the U.S. and its NATO-ally Turkey have been seriously tested by Washington’s support for Kurdish forces in Syria – but they’ve never looked quite this bad.

On Monday, Turkish President Recep Tayyip Erdoğan railed against the U.S. for building a “terror army” on its doorstep in Syria – and threatened to “strangle” the proposed 30,000-strong U.S.-backed force “before it’s even born.”

Here’s what you need to know about the latest flare-up between Ankara and Washington.

Speaking to an audience of factory workers in Ankara on Monday, the Turkish president said: “A country we call an ally is insisting on forming a terror army on our borders. What can that terror army target but Turkey?”

“Our mission is to strangle it before it’s even born.”

Erdoğan was reacting to the announcement by the U.S.-led coalition on Sunday that it was working to build a new Syrian Border Security Force to protect a swath of Syrian territory held mostly by Kurdish forces along the Euphrates River, and at the borders with Turkey and Iraq.

U.S. Army Col. Ryan Dillon, spokesman for the anti-ISIS Operation Inherent Resolve, said that half of the proposed 30,000-strong force would be drawn from the Syrian Democratic Forces (SDF), a Kurdish-dominated militia that was a key ground asset in the war against ISIS.

That’s a major issue for Turkey. It sees the Kurdish forces in the SDF – known as the People’s Protection Units, or YPG, as terrorists, considering them inseparable from the Kurdistan Worker’s Party (PKK), a banned Kurdish militant group in Turkey that has been waging an insurgency against the Turkish state for decades.

“This is what we have to say to all our allies: Don’t get in-between us and terrorist organizations, or we will not be responsible for the unwanted consequences,” Erdoğan said.

“Don’t force us to bury in the ground those who are with terrorists. Our operations will continue until not a single terrorist remains along our borders, let alone 30,000.”

In an apparent attempt to assuage Turkish concerns, the U.S-led coalition has stressed that the new force’s makeup would comprise both Kurds and Arabs, reflecting “the populations they serve, both in gender and ethnicity.”

But that hasn’t cut it with Turkey, which warned that the creation of the force will also hamper cooperation in mopping up remaining pockets of ISIS in Syria.

Despite the tough talk over the proposed U.S.-backed force, Turkey’s immediate priority is the Kurdish-held Syrian town of Afrin, which it has repeatedly threatened it is imminently preparing to target in a new military offensive. The town in northwest Syria is Kurdish-held but lies outside the main Kurdish pocket to the east that would be patrolled by the proposed border force, and has no supporting U.S. troop presence nearby.

“Erdoğan has been saying every day for a week ‘We are about to come’ and there is a sizable mobilization on the Turkish side of the border across from Afrin,” Asli Aydıntaşbaş, a senior policy fellow at the European Council on Foreign Relations, told VICE News.

The Kurds have been holding Afrin with the cooperation of Russian forces, making any Turkish offensive a test, first and foremost, of how the Russians – critical backers of the Syrian regime and the major power broker in the conflict – respond.

“The real question actually is, what will the Russians say?” said Aydıntaşbaş. “If Turkey does deliver on its threat of ‘rooting out terrorists’ from Afrin, does it mean Russians have given a green light to this move? Or is Turkey going after its own interests despite Russia and the U.S.? We do not have the answers to these questions.”

Aydıntaşbaş said that Erdogan’s threats over the border force had made it “very difficult” for the U.S. to move ahead with creating the border force. “I do think there will be an effort in Washington to appease Ankara and perhaps diminish the stature of such a force.”

But Aydin Sezer, head of the Ankara-based think tank the Turkey and Russia Center of Studies, told VICE News that he did not believe Washington would be deterred from pressing ahead with planned support for its Kurdish allies.

Nevertheless, he said, the wedge over U.S. support for the Kurds would continue to damage U.S.-Turkish relations – not that this will concern Erdoğan, as his combative stance on the issue is helping to shore up support at home.

“Turkey and the U.S. do not have any area in which we have good relations at the moment,” he said. “The radicals are saying to cut diplomatic ties with the U.S., it is to that extent. And Erdoğan likes this pressure very much.”