"More than anything, it’s the disagreements over Syria under former President Barack Obama that severely strained the U.S.-Turkish relationship. In Ankara’s view, Mr. Obama, by initially encouraging an uprising and then backing off his threats to use force against the regime of Syrian President Bashar al-Assad, left Turkey exposed to the fallout—and forced to absorb three million Syrian refugees.
Over the past two years, Ankara also seethed at the U.S. aid for PYD in northern Syria. Instead of reversing that policy, as Ankara had expected, Washington appears to be doubling down on support for the Syrian Kurdish group. In early March, the Pentagon went as far as deploying American forces between PYD and Turkish lines near the northern Syrian town of Manbij, effectively blocking a planned offensive by Turkey and its Syrian allies. Then, in recent days, the U.S.—in its most high-profile military operation in Syria so far—airlifted the PYD and its allied Arab fighters across the Euphrates, to the strategic Tabqa Dam near Raqqa."
https://www.wsj.com/articles/u-s-turkey-set-on-a-collision-course-1490866201
Over the past two years, Ankara also seethed at the U.S. aid for PYD in northern Syria. Instead of reversing that policy, as Ankara had expected, Washington appears to be doubling down on support for the Syrian Kurdish group. In early March, the Pentagon went as far as deploying American forces between PYD and Turkish lines near the northern Syrian town of Manbij, effectively blocking a planned offensive by Turkey and its Syrian allies. Then, in recent days, the U.S.—in its most high-profile military operation in Syria so far—airlifted the PYD and its allied Arab fighters across the Euphrates, to the strategic Tabqa Dam near Raqqa."
https://www.wsj.com/articles/u-s-turkey-set-on-a-collision-course-1490866201