It's against my principles to reply to this question with an answer on Sparta, since Spartans were never soldiers, but I answered a similar question focused on Sparta here.

This is a very broad and somewhat vague question; I'd recommend narrowing the scope down to a more specific time and place in history, such as the Early Roman Republic or the Han dynasty.

Although, in broad strokes, the vast majority of fighting men over human history haven't been career professionals, but instead were called from their regular professions in times of conflict, more akin to being drafted than making a living through soldiering.