"U.S. policy toward Syria since 2014 has prioritized counterterrorism operations against the Islamic State (IS, also known as ISIL/ISIS), but also has included nonlethal assistance to Syrian opposition groups, diplomatic efforts to reach a political settlement to the civil war, and humanitarian aid to Syria and regional countries affected by refugee outflows. U.S. forces deployed to Syria have trained, equipped, and advised local partners under special authorization from Congress and, as of December 2018, have worked primarily 'by, with, and through' those local partners to retake nearly all areas formerly held by the Islamic State. Following an internal policy review, Administration officials in late 2018 had described U.S. policy towards Syria as seeking (1) the enduring defeat of the Islamic State; (2) a political settlement to the Syrian civil war; and (3) the withdrawal of Iranian-commanded forces. President Trump's December 2018 announcement that U.S. forces had defeated the Islamic State and would leave Syria, signaled the start of a new U.S. approach. Questions remain about how the withdrawal will be implemented, what effects it will have on existing U.S. programs, and how U.S. priorities in Syria may change."
CRS Report for Congress, RL33487
Congressional Research Service: https://crsreports.congress.gov/