Turkey invaded Syria on Wednesday aiming primarily at the Kurdish YPG.
The YPG militia has been a key US ally against IS and controls swaths of northern Syria; Turkey considers YPG a terrorist organization.
As Turkish forces moved towards Jarablus on the banks of the Euphrates river their stated objective was to deny IS control of the border town; their unstated objective was to ensure Jarablus would not fall under the control of the Kurdish militia, paving the way for an autonomous Kurdish Syrian region.
YPG was moving to occupy Jarablus before the Turkish military intervened. Prior to the invasion, the Syrian Democratic Forces (SDF) accused Turkey of assassinating the Kurdish forces commander in Jarablus, Abdulsettar Al-Cadiri. Kurdish forces also accuse Turkey of backing Jihadists in the region