The American government has made the wrong choice right from the start and is continuing to make the same old mistakes trying to keep a presence in Syria through illegitimate and controversial means.
The Assad regime tried to take revenge for falling at odds with Ankara and thus vacated the lands bordering Turkey and allowed the PKK terrorist organization and its affiliates to take over chunks of northern Syria. Thus, Assad felt he was penalizing Turkey and its president, Recep Tayyip Erdoğan, by allowing the enemies of Turkey to control the border areas.
The PKK, which has been fighting a secessionist terrorist campaign in Turkey for the past four decades, used the Democratic Union Party (PYD) and its armed wing, the People's Protection Units (YPG), to consolidate its power in the north of Syria killing and intimidate local Kurdish politicians and thus became the tyrants of the border areas. They drove away the Kurds who opposed them and most are now housed in camps in Turkey.
The American government, instead of condemning the way the PKK grabbed power in the north of Syria and adding to the suffering of the masses who had already taken a beating from the Assad regime, opted to cooperate with the PKK. They created the alibi that they were using PKK elements in Syria to establish a fighting force against Daesh forces and helped them grab chunks of Arab towns and cities like Manbij in the north of Syria. Despite warnings from Turkey, the American government deepened their relationship with the PKK affiliates giving them 4,000 TIR loads of arms and ammunition as well as heavy arms. The American government promised Ankara that they would get the PKK affiliates out of Manbij, but that never happened. On the contrary, the PKK consolidated its hold over the city, and the U.S. seemed to encourage the PKK affiliates takeover of all the northern border areas irrespective of whether they were Turkmen or Arab regions under the pretense of fighting Daesh. The American government was in fact carving out northern Syria for the terrorists to establish an autonomous zone that would eventually become a mini-state. They tried to calm down Ankara, saying their relationship with the PKK affiliates was temporary and that the arms given to them would be taken back once the fight against Daesh was over and the Daesh headquarters in the city of Raqqa was taken.
In 2017, Turkey launched Operation Euphrates Shield that spoiled PKK plans to take over the Jarablus area and push toward the Mediterranean Sea. Some people in Turkey feel the American government has been behind the plan to give the area to PKK-affiliated terrorists on a golden plate.
Turkey has seen very clearly that the American government does not mean well. Despite repeated promises to terminate their close relationship with the PKK and its affiliates in Syria, they are pushing for closer ties with the terrorists. Now that Daesh has been effectively defeated, it is clear that the American government does not want to leave Syria and wants to hold on to their presence in this country and desires to use the PKK and its affiliates to further this goal.
Yet they are cooperating with illegitimate and questionable entities in Syria. They are in fact collaborating with terrorists.
Turkey cannot accept that and thus has decided to end the PKK presence on its border. Operation Euphrates Shield was one step. Now Turkey is aiming to clear the Afrin region of the PKK and does not want the American government to be around to protect their terrorist buddies.