President Donald Trump spoke by phone with his Turkish counterpart Recep Tayyip Erdogan on Dec. 14, 2018. The Turkish leader promised Turkey would take over security in northern Syria, and Trump agreed to bring American forces in northeastern Syria home.

Trump’s pledge caught the Pentagon and the State Department by surprise. After all, Erdogan made little secret of his desire to eradicate both Kurdish self-government in Syria and Kurdish militias. For decades, the United States agreed and acted in concert with Turkish policy, but by 2014, Turkey’s complicity with the Islamic State was too great to overlook. As ISIS besieged the predominantly Kurdish border town of Kobane in northern Syria, Turkey allowed ISIS fighters to transit its territory to resupply the organization and, in some cases, caught on video ISIS fighters who even fired into Kurdish positions from inside Turkish territory.