After the Civil War, someone asked General Pickett why the Battle of Gettysburg had been lost: Was it Lee's error in taking the offensive, the tardiness of Ewell and Early, or Longstreet's hesitation in attacking? Pickett scratched his head and replied, "I've always thought the Yankees had something to do with it." This simple fact, writes James McPherson, has escaped a generation of historians who have looked to faulty morale, population, economics,...