Turkey's military operation in Syria, which is backed by US air strikes, is targeting ISIS but its main goal is to prevent the creation of a Kurdish-controlled territory on the other side of Turkey's border.
The YPG Syrian Kurdish force is one of the most powerful militias in Syria and regarded as the backbone of the Syrian Democratic Forces (SDF), a U.S.-backed alliance formed last October to fight Islamic State.
Turkey views the YPG as a threat because of its close links to Kurdish militants waging a three-decade-old insurgency on its own soil. It has been alarmed by the YPG's gains in northern Syria since the start of the Syrian civil war in 2011, fearing it could extend Kurdish control along Turkish borders and fuel the ambitions of Kurdish insurgents in Turkey.
Turkey also views the YPG and its political wing the Democratic Union Party (PYD) as being an offshoot of the Kurdistan Workers' Party (PKK), which has been engaged in a long-running guerilla war with the Turkish state and has been responsible for many terrorist attacks inside Turkey resulting in the deaths of many Turkish police officers, soldiers and innocent civilians.
Turkey's stance has put it at odds with Washington, which sees the YPG as a rare reliable ally on the ground in Syria, where Washington is trying to defeat Islamic State while also opposing President Bashar al-Assad's government in a complex, multi-sided, five-year-old civil war.
Just days before the Turkish incursion into northen Syria, U.S.-allied Kurdish forces successfully overtook the former ISIS stronghold of Manbij.
Jarablus and Manjib to the south were liberated from Isis fighters by Kurdish-led forces are considered essential to connecting the western and eastern semi-autonomous Kurdish areas in Syria.
But now the Turkish offensive managed to throw out the same Syrian Kurds and replace them with Turkish backed Syrian rebels.
Turkish jets also carried out airstrikes on YPG/SDF positions south of Jarabulus, which was captured by Turkey and its allies on Thursday from IS.The clashes are likely to be a headache for the US, who ostensibly support both sides involved in the fighting.
The incursion comes at a testing time for Turkish-U.S. relations. Erdogan wants the United States to extradite Fethullah Gulen, a Turkish cleric who has lived in self-imposed exile in Pennsylvania for 17 years and whose religious movement Turkey blames for staging last month's failed coup.
If the situation in Syria has been described as being "complicated", it's going to get a lot worse with a full blown conflict erupting in northern Syria between Syrian Kurds and Turkey, both backed by the US.