John Price Jones 
German Secret Service in America 1914-1918 [PDF ebook] 

A nation at war wants nothing less than complete information of her enemy. It is hard for the mind to conceive exactly what "complete information" means, for it includes every fact which may contain the lightest indication of the enemy strength, her use of that strength, and her intention. The nation which sets out to obtain complete information of her enemy must pry into every neglected corner, fish every innocent pool, and collect a mass of matter concerning the industrial, social and military organization of the enemy which when correlated, appraises her strength-and her weakness. Nothing less than full information will satisfy the mathematical maker of war.