The convoy rumbled into the Taliban heartland, a white desert littered with stones. Over the loudspeakers at the local mosque, the Afghan police officers ordered everyone to gather: The commander was here. Dozens assembled in the mud square to listen as Abdul Raziq, one of America’s most important partners in the war against the Taliban, stood before the crowd, gesturing at two prisoners he had brought along to make his point. The prisoners knelt with their hands bound as Raziq spoke to...