This book attempts to present an organizational history of Military Intelligence in the United States Army from its beginnings to the present. "Military Intelligence- the collection of information by commanders on the enemy and the battlefield environment they must confront- has existed since the beginnings of armies and of wars. However, the emergence of professional Military Intelligence organizations and the definition of the functions they most appropriately performed are comparatively recent developments. Until the nineteenth century, Military Intelligence was practiced only in wartime; methods of collection were rudimentary; and the conduct of Military intelligence was considered a function of command, one which any professional officer could perform. Furthermore, commanders tended to be skeptical about the reliability of the information they received from spies, scouts, and their own troops. In his monumental treatise On War, Clausewitz commented only that "many intelligence reports in war are contradictory, even more are false, and most are uncertain."
United States. Dept. of the Army: http://www.army.mil