This article talks about the US as monolithic, but my limited understanding is that although SDF's key ally was the US Army, CIA had clandestinely been backing a different Syrian faction, so "the US" has never wholeheartedly been onside throughout. (and, presumably, now it's State's turn to be backing Erdogan? Pawns will be sacrificed.)

'America has no permanent friends or enemies, only interests' — HAK