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The turbaned Taliban gather in front of the entrance to the deserted barracks. They fish their smartphones out of their vest pockets, switch on their flashlights, and illuminate the ghostly darkness inside. There are no windows in the corridor, nor the small, neat rooms to the left and right. If it weren’t for the nice office chairs and the comfortable bathroom cubicles, one might mistake them for cells. But the rooms are part of a section of a now abandoned military base in Shkin, in southeastern Afghanistan, that was once occupied by U.S. forces, including CIA officers. Now, with the base empty and the Taliban back in power, Washington’s ability to monitor the assorted militants who are still in the area has become far more limited.

Shkin lies on a godforsaken high plateau framed by ridges of forested hills less than five miles west of the Pakistani frontier, the border denoted by the colonial-era Durand Line th…