 Complete the following phrase:  The Syrian Democratic Forces are an armed participant in the Syrian Civil War and serve as the armed forces of the Autonomous Administration of North and East Syria. The SDF is composed of numerous groups, most prominent among them being the YPG and YPJ and their political branch, the PYD, which Turkey considers a branch of the Kurdistan Workers Party PKK, an organization which Turkey considers a terrorist group and with which it has engaged in armed conflict since the breakdown of peace negotiations in 2015. For this reason, Turkey views the entire SDF as nothing more than an extension of the PKK. This has led Turkey to intervene twice against the group, first by invading Northern Syria to prevent the linking of SDF-held areas and later by starting 