When Turkey’s President Recep Tayyip Erdogan threatened to attack the United States’ Kurdish allies in northeastern Syria, the US-led coalition would often respond by dispatching a mobile patrol prominently displaying the US flag within sight of Turkish troops across border. On Sunday, Turkish and US troops conducted their first joint ground patrol in and around Tell Abyad, a mainly Arab town that until its 2015 capture by the Syrian Kurdish People’s Protection Units (YPG) served as a critical gateway for thousands of foreign jihadist fighters flowing into Syria via Turkey.

The exercise marks the first time Turkish troops crossed into the US-protected and Kurdish-administered zone in northeastern Syria to the east of the Euphrates River and is part of ongoing efforts between Turkey and the United States to establish a so-called security mechanism that many Syrian Kurds worry may torpedo their own.

In Ankara’s eyes its overarching purpose is twofold: to pressure the United States into severing its partnership with the YPG as soon as possible and to create a safe zone running ideally from another Arab-majority town, Manbij, all the way to the Iraqi border. Ankara wants to send back up to a million of the estimated 3.6 million Syrians sheltering in Turkey who have become a huge political liability for Erdogan.

For Washington, the security mechanism is meant to avert a Turkish invasion and protect its allies in the YPG-led Syrian Democratic Forces (SDF) so they can, in the words of coalition spokesman Col. Myles B Caggins III, “remain focused on the enduring defeat” of the Islamic State (IS). In a statement, Caggins noted that Turkish forces had “observed first-hand progress on destroyed YPG fortifications and areas where YPG elements voluntarily departed the area.”