Washington realizes that Ankara is getting out of control and a robust military fist along the borderline will only assist Erdogan. U.S. support for the SDF has put enormous strain on ties with NATO ally Turkey, which views the YPG as an extension of the Kurdistan Workers’ Party (PKK) - a group that has waged a three-decade insurgency in Turkey and is considered a terrorist group by the European Union, Turkey, and the United States.