In 2013, after months of secret negotiations between imprisoned Kurdistan Workers Party (PKK) leader Abdullah Ocalan and Turkish leader Recep Tayyip Erdogan’s representatives, Turkey and the PKK agreed to a cease-fire, one condition of which was the departure of PKK fighters from Turkey. As part of that agreement, many entered Syria where they augmented the People’s Protection Units (YPG), the most successful indigenous force first in the fight against the al Qaeda-affiliated Nusra Front and then in the battle against the Islamic State.

Indeed, it was U.S. recognition that the YPG were fighting against ISIS while Turkey was passively if not actively supporting ISIS that led the Obama administration to start partnering with the YPG over the objections of Turkey.