[Anthony]

Originally Posted by DanHoward

A mercenary is a soldier who serves in a foreign army.

That doesn't appear quite right. Checking the Geneva Conventions, we get:

1. is specially recruited locally or abroad in order to fight in an armed conflict;
   So if you show up and offer services, not a mercenary?
2. does, in fact, take a direct part in the hostilities;
   Not a mercenary if you don't actually fight. I guess they're classed as non-combatants if they don't?
3. is motivated to take part in the hostilities essentially by the desire for private gain and, in fact, is promised, by or on behalf of a Party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that Party;
   Unclear what this means if the Party does not have regular armed forces.
4. is neither a national of a Party to the conflict nor a resident of territory controlled by a Party to the conflict;
   That approaches your definition, though it's not specific to nations.
5. is not a member of the armed forces of a Party to the conflict; and
   So you can recruit foreigners all you want as long as you actually include them in your own armed forces.
6. has not been sent by a State which is not a Party to the conflict on official duty as a member of its armed forces.
   So 'military advisors' are not mercenaries. Even if paid.