Turkey has initiated a military operation into Northern Syria. The operation follows a U.S. decision to withdrawal from the immediate area controlled by the Syrian Democratic Forces (SDF) - the primary Syria-based U.S. partner in the campaign against ISIS. As Turkish forces move across the border and the SDF responds, ISW's Syria expert John Dunford explains the context, key dynamics shaping the battle, and the effects on the counter-ISIS campaign. *Originally published October 9, 2019.