Following at least two days of heavy bombardment and shelling, Turkish forces and their Syrian rebel allies have begun the ground phase of their invasion into the northern Syrian region of Afrin — dubbed Operation Olive Branch. The advance against the Kurdish People's Protection Units (YPG) that control the region has so far been slow but methodical, inhibited by the area's difficult and mountainous terrain. Nevertheless, as of Jan. 22, Turkey and its allies have already seized a number of strategic hilltops, mountain peaks and villages. The majority of the advancing forces have come from north of Afrin, and the predominantly rebel formations on the eastern side of the battle zone have now launched their own assaults on the Kurds. Most of the difficult terrain is concentrated around the current frontlines, meaning that if Turkey and its allies accomplish their initial objectives they will have an easier task advancing farther.