The Wikipedia entery on Nom de Guerre

Nom de guerre

Pseudonyms are adopted by resistance fighters, terrorists and guerrillas often to make enquiries more difficult, to create and maintain an aura of mystery, and to protect their families from reprisal, although other reasons often may exist. The expression nom de guerre (/nɔ̃ də ˈgeʀ/, "name of war") is often used for such pseudonyms, though this expression is rarely actually used in French. It is occasionally used as a stylish substitute for nom de plume.

Noms de guerre were frequently adopted by recruits in the French Foreign Legion as part of the break with their past lives. Pseudonyms used by some members of the French resistance were integrated into their last names after World War II; for instance, Jacques Delmas, alias Chaban, became Jacques Chaban-Delmas.

Another famous nom de guerre is Willy Brandt, adopted in 1934 by a German resistance fighter named Herbert Ernst Karl Frahm who had fled Germany for Norway. After his return to Germany, he had the name Willy Brandt officially recognised. He later became mayor of West Berlin, West German foreign minister and West German chancellor under that name.

The legends of Robin Hood, among others, frequently indicate that the names of Robin and his Merry Men are not their "real names"; for example, Robin was supposedly born as Robert Fitzooth or Robert of Locksley. In legend as in history, outlaws may call themselves by noms de guerre to avoid identification by those who outlawed them.

Within Communist parties and Trotskyist organisations, noms de guerre are usually known as party names or cadre names. This took hold because revolutionaries were often persecuted by states (and also, in the case of Trotskyists, by pro-Soviet communist parties).