In his state of the union address Jan. 28, President Obama said that when it comes to fighting terrorism, "America must move off a permanent war footing." In 2013 he took steps to wind the war down, cutting drone strikes and drafting a replacement to Congress's broad authorization for the conflict. But just as waging the war has been messy, ending it is too.
Consider the case of Sabir Ali Khan. In 2010, al-Qaeda and Taliban attacks killed 39 Americans in Afghanistan's Kunar province, and Khan was one of the conspirators, according to U.S. court documents. He had smuggled mortars across...