How did mercenaries operate for most of history?
I got to pondering the nature of mercenaries. Sometime back I was autistposting about the different types of military service (in reference to Victoria 3), that with some level of abstraction soldiers can be thought of as voluntary or conscripted (and that conscription for life, as with janissaries, strelets and mamluks, or for a limited term) and their units can be raised on permanent (standing army) or temporary (mobilization) basis. And I said how a really badass Victorian/Cold War/contemporary game would reflect the meaningful differences between the three by tying it into a political system that ties into the diplomatic system, because your pool of volunteers (especially for mobilization-only units) could be made to depend on how the public feels about your wars and how they feel about your wars can be different for different demographics, and even to have potential for foreign recruitment as well (think Abraham Lincoln Brigade, for example).
I also very much like the idea of it being possible to have army units that can be mobilized and demobilized, basically "mothballing" an army, so you can actually choose what kind of conscript army you want to raise, how much of it to raise, but also to be able to organize it all in advance, even having a preset that you can flip a switch (or even from a menu of presets) so you just plan your mobilization in advance.
Anyways, as it goes with mercenaries, the whole concept of a mercenary is mighty vague. A soldier for hire, of course, but what does that really mean? Mercenaries can be temporary, but so can they be permanent (Varangian Guard). They can be foreign or they can be domestic. They can even be coerced, seeing as armies of the Thirty Years War frequently impressed civilians into their forces. And at the end of the day, every peacetime soldier is a soldier for hire, even if they have other motivation too.
What I see the mercenary as really is a contractor. That, at the scale of a Paradox game, their defining characteristic is that they preexist as units and are hired on without being "your" units. And that's how CK2, Imperator: Rome, and I think now EU4 operate, though EU4 used to have them as just "money instead of manpower" units.
So how did these things operate? I have no clue. Paradox games depict them sitting on their asses as huge raised armies until someone calls them to service. Did they really do that? It seems like that would be horribly ineffective in unpredictable world markets, and they'd be a huge liability for any ruler to let them sit around in their land. I would think that maybe what mercenary companies really did was they'd have a core of officers, bosses, that were more or less on retainer, maybe not that formal but definitely reliable to each other to be available, and the mercenary company would, if not mobilized, go on a big recruiting campaign. But I have no idea how, in an organizational sense, this stuff was done, maybe my guess is completely wrong.
In my imagination, there'd be supply and demand for mercenaries. The more of an armaments industry a place has, the more young-skewed its population structure, the more its institutions restrict options for social mobility or even normal life outside of raiding, the more fucked up and devastated the society is, the more recruitable men there should be. Then, the companies would have two types of reputation. A reputation for its men of being a worthwhile "employer" - that it has an excellent revenue to casualty ratio - and a reputation to the AI nations for being a winner, perhaps effecting a sort of reservation wage. So successful companies expand by being able to mobilize more men when they need to and also are more expensive. During a mobilization of mercenaries it should be possible to engage in a bidding war for the mercenaries. Did that happen, historically? I would think that mercenaries would have held out to see who was going to offer better terms.
Then, lastly, there ought to be a way to bribe an army to defect. That may be the single most interesting feature of the mercenary, the way they could break empires on a flip (like Manzikert), but it can't happen in a Paradox game. It'd be like a reputation hit to the company and require a much bigger lead than bidding for the initial contract, but would be a possibility.