Turkey has long said YPG is the Syrian offshoot of the PKK, an internationally recognised terror group. But Washington, Ankara's close ally, plays down links between the two groups and continues to support the YPG in Syria.
After months of battle, US-backed Syrian Democratic Forces (SDF) ousted Daesh from their self-declared capital in Raqqa, Syria last year.
The SDF is back under the spotlight, with YPG – its major force – in the crosshairs of the Turkish military.
Ankara aims to clear the Turkish-Syrian border of the YPG, which it sees as linked to PKK, designated a terror group by Turkey, the US and the EU. But the US sees the YPG as essential in its fight against Daesh.
TRT World's Alican Ayanlar takes a closer look at the link between the PKK and YPG, and how Operation Olive Branch could raise tensions between Turkey and the United States.