I've the last segment of a campaign coming up in 2 months and we're looking to add a layer of mechanics as a resource for the players and to help track/adjudicate progress.
Long story short, the players were part of a crusade that fell apart. The players went off on their own and have resurrected a god and restored a portion of their religion. Now they're going back to the mortal plane and will be attempting to build a coalition army to vanquish their foes.
I'm looking for suggestions of any game/supplement that handles this in a mechanical fashion. Please limit suggestions to things that are in print or are easily obtainable.
Not looking for mass combat rules, or single scene diplomacy/intrigue rules, but rather long term diplomacy rules, ideally stuff that operates on the scale of months or years.