The Twilight rules, going back to the 1980s, presume that most player characters either started the war as active-duty military, reservists, or civilians who were drafted into the military at the beginning of hostilities.
Other scenarios - "Merc 2000" among others - presume the characters join the campaign directly from whatever career they were involved in up until the beginning of the scenario.
But in many nations, military service is to some degree or another a part of citizenship. This varies by nation:
Some nations, like Israel, expect citizens to serve a fairly significant period on active duty, and then remain in the active reserves for a period of time.
Others - Finland, Switzerland, Singapore - require citizens (usually male) to undergo basic and advanced training, and sometimes a short period on active duty, before a number of years "reserve" service (usually amounting to a number of days of training per year, or over the course of a number of years, before serving in organized reserve units.
Still others will require basic training and a short period of active service, after which most characters will only see military service in a major emergency.
The characters will then carry on with civilian careers, until mobilized for the war (in the traditional Twilight scenario) or, at referee's discretion, apply the skills in whatever campaign is started.
National service type is called out in the various national details.
These characters will generally serve over a year on some form of active duty, before joining the reserves for a number of years (as specified in in the character's nation's details).
These characters may start a civilian career during their first term, but receive no secondary skills for the first term.
These characters receive Basic Training and First Term skills in the applicable military career of their choice. They roll for initiative as reservists.
These characters will generally spend 6-12 months in training and on active service before joining the reserves for a number of years (as specified in in the character's nation's details).
Troops may begin a civilian career, but spend one term's worth of secondary activities getting the following from the service branch of their choice:
Basic training skills
One First Term skill of their choice a level 1, and the rest at Level 0.
They roll for initiative as reservists.
These characters will generally have less than six months of training and nearly no active service before joining the reserves for a number of years (as specified in in the character's nation's details).
They receive their chosen service's basic training skills, and receive first term skills at level 0.
They roll for initiative as reservists.