Turkey has initiated a military operation into a part of Northern Syria controlled by Syrian Kurdish-led forces. The operation follows a U.S. decision to withdraw from the immediate area controlled by the Syrian Democratic Forces (SDF) – the primary Syria-based U.S. partner in the campaign against the Islamic State of Iraq and al Sham (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.