Most histories of the Civil War explain victory or defeat in terms of the skill of commanders, the fighting qualities of the troops, and resources in men and materials. Intelligence has been largely ignored because so little was known about it. Fishel's book is the first to examine in detail the impact of intelligence, and for virtually every campaign this intelligence explanation alters, sometimes radically, the historical picture.

Most histories of the Civil War explain victory and defeat in terms of the skill of commanders and their troops. Intelligence records disappeared after the war, and thus a critically important element has largely been ignored. Fishel has unearthed substantial collections of such records, and his "intelligence explanation" radically alters history's understanding of the campaigns. The Secret War for the Union is one of the most important Civil War works ever published.