When President Obama, without congressional authorization, ordered U.S. forces to intervene in Syria, the mission was clear: liberate ISIS-held territory. That mission has been achieved. Leaving behind U.S. forces in Syria involves large risks without any security upside: it threatens to drive adversaries into allying against the United States; to inflame Islamist-nationalist sentiments in Iraq and Syria, making U.S. forces targets; and to risk U.S. conflict with Russia, Iran, Turkey, and Syria for no good reason. Staying also ties down U.S. forces and limits their focus on core missions. With ISIS’s “caliphate” destroyed, U.S. troops have achieved all they reasonably can and should be fully withdrawn.