From the early 15th century: "military campaign; the act of rapidly setting forth," from Old French expedicion "an expediting, implementation; expedition, mission" (13c.) and directly from Latin expeditionem (nominative expeditio) "an enterprise against an enemy, a military campaign," noun of action from past participle stem of expedire "make ready, prepare." I present this as an alternative to the term, "military operations," which did not come into use until 1749.
I don't know that there's a single DM reading this who would either feel up to the task or even be willing to have something like this, long term, be a part of their role-playing game . . . but let's pretend that it's a possibility. Let's pretend that your 12th level characters can pull together an army, between them, of a few thousand. Let's pretend the DM has the wherewithal to do more than just have you write off gold pieces to bivouac them, train them, organize them into battle squadrons (1560s) and march them out with the express purpose of conquering a part of the world. Let's pretend that the DM is willing to let this - we'll call it a 'war' - continue over several seasons in time, in which more soldiers are found and trained, more ground is taken or lost and that the possibility of winning or losing is in doubt, enough doubt that the experience itself actually causes a sense of engagement that lasts for months of running.
We'll have to pretend because we know there are NO rules for this. We know that the set up that exists in most worlds comes down to the DM rolling a d6 and stating - pretty much ad hoc and without much personal desire to see the thing through - that we've won or lost. We also know, alas, that because the Gygaxian crowd never considered the possibility that players might imagine themselves running a lengthy campaign as Caesar, Belisarius or Jenghis, with zero interest in dungeon-delving, such is not considered an acceptable way to play the game.
Let me repeat that. We're in the wrong if that's what we want our players to do. That's not what D&D was designed for. If that's what you want, there's a war gamer group right down the road.
That's why we have to pretend; because we can't role-play that shit.
Here I want to point out a fundamental difference between 'wargaming' and 'role-playing.' In wargaming, the ideal is to measure ourselves against what Caesar, Belisarius or Jenghis did. They won or lost such-and-such a battle, we want to see - given an approximate simulation - if we will make the decisions that will enable us to win or not. We know what those conquerors tried; we want to see if our change to their tactics improves the situation, if we can think of something they did not or prove to ourselves (dissonantly) that we're the equal of them.
This is nothing like role-play. When I roll a die as a fighter, my actual personal ability as a fighter is immaterial. The fighting ability is inherent in the statistics of my character, not in me. I am playing my character to see if his stats and luck will win out over some other creature or character's stats and luck. Strategy comes into play, of course, but I have the benefit of many things that are imaginary and unreal, very much making this situation different from a simulation. No RPG battle ever occurred in the real world.
This means that if we make rules for running a campaign in D&D, it isn't just a matter of inventing rules for large unit battle resolutions, which I can then run like any other war game. In D&D, I am entitled, also, to my fighter's superior understanding of war and the movement of men in ways that I do not possess! I don't want to be limited to my own ability to develop and organize an expedition, nor do I want any player in my campaign to feel similarly bound. If they are a total wash as a wargamer, in role-playing they should have the benefit of being able to win just like any other world-class conqueror.
I am guessing I'm probably the first person in the history of the game to make this point.
It means that rules for running an expedition must include a reasonable chance for success regardless of the player's personal military understanding or experience. A know-nothing player can roll a die for their fighter and kill a dragon; rules for military expeditions must possess that same fundamental gaming structure.
A very tall order. I am still thinking about it.