While the worst violence of Syria’s six-year-long war may be over, a credible settlement is still far off. The Trump administration has boosted the U.S. military presence in Syria. While this enhances Washington’s leverage, it will not alone assure long-term U.S. interests: The destruction of the Islamic State, a cessation of Syria’s destabilizing refugee flow, and an end to Syria’s transformation into a logistical hub for expanding Iranian and perhaps Russian influence. To achieve these aims, Washington needs a more holistic approach toward external powers now involved in the Syrian conflict. But much of U.S. policy will hinge on the influence of other regional players, Iran and Turkey. Both have vested interests in Syria that do not align well with those of the United States, and the Trump administration will have to manage these unreliable actors carefully as the conflict continues.