It's an incredibly strange thing, from a historical perspective to have any distinction between the term citizen and the term soldier. It was, for every definition of the term citizen until 1973 (or perhaps until 1920), an obligation of all citizens that they fight when their country (or cit-y) was at war. This is why they were male, why the bearing of arms was so important (because depending on the time, you'd bring your own), and why civic spirit is high in such societies. Citizen societies embrace the cameraderie of a war band, whose freedom is guaranteed by their own hands. This is why the disjunction between citizen and slave existed, because citizens were those people, or the descendents of those people who guaranteed their freedom, and their property (land, cattle, etc.) by force of arms.

Now it's just a little blue passport book.