The secretary has a chance to turn the deteriorating relationship around if he can convince Ankara that the U.S.-YPG alliance is winding down.
Secretary of State Rex Tillerson's trip to Turkey offers a reminder that while Turks are divided 50-50 on many political issues—including their feelings toward President Recep Tayyip Erdogan—they broadly oppose the U.S. policy of working with the Kurdish People's Protection Units (YPG) in Syria. Most Turks object to U.S. partnership with the YPG—an offshoot of the Kurdistan Workers' Party, which is viewed as a terrorist group by Turkey and its NATO allies. Secretary Tillerson needs to assure the Turks that the U.S. policy was a temporary and tactical alliance, and that Washington is ready to wean itself away from the YPG now that the Islamic State has been defeated in Syria.
Washington shouldn't worry too much about upsetting the YPG with such a shift in policy. The YPG seeks international legitimacy for the areas it controls in Syria, collectively dubbed Rojava. Yet the YPG knows that a split from the U.S. would force Rojava to fold under Moscow, becoming a Syrian version of Transnistria—Moldova's breakaway republic now under Russian military occupation and lacking any international legitimacy.