excerpt from Wikipedia
Noms de guerre
In Ancien Régime France, a nom de guerre (a French phrase meaning "war name") would be adopted by each new recruit (or assigned to him by the captain of his company) as he enlisted in the French army. These pseudonyms had an official character and were the predecessor of identification numbers: soldiers were identified by their first names, their family names, and their noms de guerre (e.g. Jean Amarault dit Lafidélité). These pseudonyms were usually related to the soldier's place of origin (e.g. Jean Deslandes dit Champigny, for a soldier coming from a town named Champigny), or to a particular physical or personal trait (e.g. Antoine Bonnet dit Prettaboire, for a soldier prêt à boire, ready to drink). In 1716 a nom de guerre was mandatory for every soldier; officers did not adopt noms de guerre as they considered them derogatory. In daily life, these aliases could replace the real family name.
Noms de guerre were adopted by members of the French resistance during World War II for security reasons. Such pseudonyms are often adopted by military special forces soldiers, such as members of the SAS and other similar units, resistance fighters, terrorists, and guerrillas. This practice hides their identities and may protect their families from reprisals; it may also be a form of dissociation from domestic life. Some well-known men who adopted noms de guerre include Carlos the Jackal, for Ilich Ramírez Sánchez; Willy Brandt, Chancellor of West Germany; and Subcomandante Marcos, the spokesman of the Zapatista Army of National Liberation (EZLN). During Lehi's underground struggle against the British in Mandatory Palestine, the organization's commander Yitzchak Shamir (later Prime Minister of Israel) adopted the nom de guerre "Michael", in honor of Ireland's Michael Collins. Revolutionaries and resistance leaders, such as Lenin, Trotsky, Golda Meir, Moshe Dayan, Philippe Leclerc de Hauteclocque, and Josip Broz, often adopted their noms de guerre as their proper names after the struggle. George Grivas, the Greek-Cypriot EOKA militant, adopted the nom de guerre Digenis (Διγενής). In the French Foreign Legion, recruits can adopt a pseudonym to break with their past lives.
Nom de guerre is now ordinary French for a pseudonym - being used in many contexts where a more specific term would be used in English.