The US began to help create the SDF in 2015. The US also had the Iran deal the same year and the Turkey-PKK ceasefire ended that year and Russia intervened in Syria. This made that year a turning point. Turkey had two elections. Another turning point arrived in 2016 as Turkey had a coup and then launched Euphrates Shield, the first of several operations in Syria the culminated in Idlib in 2017, Afrin in 2018, Tel Abyad in 2019 and then Idlib in 2020. That is how Turkey began to use the Syrian rebels to fight the US-backed SDF. It formed the rebels into dependent proxies in 2016.
What do we know about Turkey’s claims that it should have been chosen by the US to liberate Raqqa. In fact Turkey did little to help the Syrians as Aleppo fell in 2016. It wanted to focus on the YPG.
We know this because of Ash Carter’s long article at the Belfer Center. He writes:
Turkey was less interested in fighting ISIS than in preventing Kurds in Eastern Syria from linking up with those in the town of Afrin, less than 100 miles away from Manbij, which would give the Kurds control of a continuous stretch along the Syria-Turkey border. Turkey grudgingly agreed to plans for the Syrian Arab Coalition, which operated under the umbrella of the Kurdish-led Syrian Democratic Forces, to seize Manbij, just 25 miles from the Turkish border. The Turks demanded assurances that Kurdish elements of the SDF would immediately withdraw to their positions east of the Tigris River once Manbij was freed. When the SDF didn’t move as quickly as the Turks wanted, Turkey’s own proxy forces began shelling SDF positions. Likewise, Turkey regularly and publicly erupted at the thought of the SDF leading the drive on Raqqa. Turkey claimed—in the absence of any evidence—that fighters it supported could take Raqqa instead. But the Turks could never produce an actual plan to field such forces in repeated meetings with the U.S. military. Turkish-backed forces in Syria in fact struggled to take less heavily defended objectives much closer to its border, such as Al Bab. Turkey was also a problem in Iraq. It established a base north of Mosul and conducted air strikes against its Kurdish opponents in Iraq, which understandably angered the Iraqi government.
This reveals that Turkey had sought to work with the US but that the concept fell through. Was that due to the inadequate rebels? The US had funded and trained rebels but the program was wrapped up over time. We may never know. But the Belfer Center has a map that shows a concept for the plan. It never happened.