In early 2015, the group won a major victory over the Islamic State of Iraq and the Levant (ISIL) during the siege of Kobanî, where the YPG began to receive air and ground support from the United States and other Combined Joint Task Force – Operation Inherent Resolve militaries. Since then, the YPG has primarily fought against ISIL, as well as on occasion fighting other Syrian rebel groups and the Turkish Armed Forces. In late 2015, the YPG became part of the SDF, an umbrella group intended to better incorporate Arabs and minorities into the war effort. In 2016–2017, the SDF's Raqqa campaign led to the liberation of the city of Raqqa, the Islamic State's "de facto" capital. Several western sources have described the YPG as the "most effective" force in fighting ISIL in Syria.