Fun with the Cuban revolution

As I continue the long, hard slog through Che Guevara: A Revolutionary Life (approaching Page 300), another fact:

Fidel Castro's nom de guerre was Alejandro.

1. Why do people have noms de guerres? Are they just fancy code names, or is there a difference between a code name and a nom de guerre? Because I imagine that a nom de guerre is mostly for fun. "Let's play war. I'll be "Alejandro."

2. Alejandro is part of Castro's real name, so way to be creative, Mr. Revolutionary Leader Turned Dictator.