Ok from your original post and your comments since it sounds to me that what you're actually looking for from a game-play perspective has very little to do with D&D. Most of D&Ds rules revolve around the player's running individual character's in small scale combat.Those are the stories that suit the system best. When we are playing the the hero's or the villains or the ultimately doomed fools involved in a personal life or death struggle is when D&D really shines. The rest of the system is designed to help the DM and the player's make the world, the characters and the fight itself more meaningful or fun for everyone involved. What you're looking for is not a D&D adventure because the rules for D&D are not built for the game you want to play. What you are speaking about is an entirely different game, something which has social interaction, political intrigue and military resource management as the primary sources of engagement for both role playing and game-play but set in a D&D (or at least high fantasy) world. You don't want to be a member mismatched group of unlikely adventures sent to go stop the evil overlord's high priest from summoning cthulhu's third cousin lulutoth and thus winning the war for the forces of darkness. You want to play a member of the war council that decides who gets sent to kill the high priest, and whether or not to try to poison the enemy giant’s or bribe them to your side ect. That sounds pretty interesting to me actually but D&D's social interaction and non-adventure based storytelling rules are bare bones compared to a lot of other rpgs. Obviously you could run a campaign like the one you describe in D&D system, that the beauty of role playing games you can do whatever the nine hells you very well please, but it would not be “optimal”. If I were you I would look into something like FATE for rules light social role playing potential and maybe make you’re own rules for the army management and warfare if none of the editions mass combat rules or a WH40K like setups don’t sound like they fit.I can think of a few other’s ways this could be done or some changes to the type story you’re looking to experience that would make D&D a better system for a lot of the game but this post is long enough.

TLDR: read bold text above