From the 1950s through the 1970s, Esperanto was used by the US Army for war games, as the language of the "Maneuver Enemy", also known as the "Aggressor". The goal was an added touch of realism, with the enemy speaking a "foreign" language. Spanish was the original candidate for Aggressor's national language, but it was soon superseded by Esperanto.
Ĉu vi parolas la Agresantan?
The Smoking Gun: Esperanto - The Agressor Language, several scanned pages
Military Info Publishing - Manuals and such, offers the Esperanto Aggressor manual
Dictionaries (Military) Photocopies - Esperanto Aggressor manual is here
Wargaming Military Manual Photocopies - A collection of Aggressor manuals is here
Army Forces Patches History 2 - Look for Aggressor
Field manuals of particular interest...