Though much has been written about the Civil War itself, little has been written about the spy war that went on within.

Each side still used age-old intelligence techniques, such as code-breaking, deception, and covert surveillance. However, into this modern war came two innovations that would endure as tools of espionage: wiretapping and overhead reconnaissance.

What follows is a look at some of the highlights of how the North and the South gathered and used their information, the important missions, and the personalities. From this special view, the focus is not on the battlefield, but on a battle of wits.