A Syrian rebel fighter stands north of Aleppo and watches smoke billow from Afrin, where Kurdish People's Protection Units are facing off against Turkish and Turkey-backed fighters. 

After days of bombarding Kurdish positions, Turkish forces and their Syrian rebel allies began a ground invasion into the northern Syrian region of Afrin. Dubbed Operation Olive Branch, the invasion targets the Kurdish People's Protection Units (YPG) that control the region.

Turkish President Recep Tayyip Erdogan gives a speech on Jan. 26. The YPG are central to the U.S. strategy against the Islamic State. Now Washington is afraid of a clash between its military and Turkish-backed forces.

A soldier stands in front of Turkish army tanks preparing to enter Syria on Jan. 21. The majority of the advancing forces have come from north of Afrin, though rebel forces have launched their own assaults from the east. 

A Turkey-backed Syrian rebel fighter launches a drone in the Afrin region, near the Turkish border. 

Debris from fighting covers the Kurdish town of Jandairis in northern Afrin on Jan. 24. The YPG has rushed to reinforce their defenses in the area, as Turkey ramps up its military campaign.

Syrians who fled Jandairis arrive in Afrin city Jan. 25.

Syrian women attend a funeral for civilians and military personnel killed during the operation in Afrin. The most difficult terrain is concentrated around the current front lines. If Turkey and its allies accomplish their initial objectives, they will have an easier task advancing farther.

A member of the Syrian Democratic Forces stands guard in Raqqa in October 2017, after taking the city from the Islamic State. The YPG forms an integral part of the U.S.-backed forces battling the Islamic State.

Members of the U.S.-backed Syrian Democratic Forces prepare to deploy to the outskirts of Manbij in March 2017. Turkey's operations could extend into Manbij, east of Afrin, but that move would not be accepted lightly by the United States. 

A Syrian rebel fighter stands north of Aleppo and watches smoke billow from Afrin.