Many histories of the American Civil War explain victory or defeat in terms of the skill of commanders, the fighting qualities of the troops, and resources in men and material. Intelligence has often been ignored, because at the end of the war most of the intelligence records disappeared. They remained hidden for almost a century, but the author of this book uncovered them during 40 years of research. The book examines the impact of intelligence on the Civil War, and as a result offers a revised historical picture of the various campaigns.