Emmanuel Debruyne, "Espionage", dans Ute Daniel et al. (éd.), 1914-1918-online. International Encyclopedia of the First World War, Berlin, Freie Universität Berlin, 2014.
During the entire First World War, warring powers used the “secret war” to try to break the balance of the battlefield. Generally created in the previous few decades, intelligence and security services saw strong development during the war: the warring sides were committed to espionage behind enemy lines and in the neutral countries, but also performed other tasks such as tapping radio communication; sabotage; counterintelligence; and propaganda. The secret war was also fought in the mind, as all warring societies were consumed by spy mania, and began to recognize their own spies as true heroes and heroines.