The Battle for Raqqa and the Challenges after Liberation | Combating Terrorism Center at West...

"The failure of the U.S. program to train and equip the Syrian rebels to fight the Islamic State coincided with the growing influence of the YPG beyond Kurdish areas. This put the United States in a difficult position as Turkey began to see the expansion of the Kurdish militia as a threat to its national security. Turkey also feared that weapons given to the YPG could be used against it by the PKK.21 The reluctance of the Obama administration to ally itself with the Syrian Sunni Arab rebels, combined with the skills the YPG developed in combating the Islamic State, enabled the YPG and allied forces to become an indispensable force in the fight, even beyond Kurdish areas."