AP Photo

Decked out in dress greens, his uniform so laden with insignia, badges, patches, ribbons, and medals that it seemed to pull him into a slight stoop, the Most Important General in America, David Howell Petraeus, arrived on Capitol Hill in September of 2007 bearing remarkable news.

Just back from Baghdad, the hot center of a four-year-long war that had come to be seen as a fiasco, Petraeus would testify that things had begun to improve—that the counter-insurgency strategy he had initiated eight months before was working, against all odds and expectations. Violent incidents had fallen off dramatically. Former Sunni insurgents had come around and begun to oppose al-Qaeda. Dangerous Shiite militias were putting down their arms. Instead of...