I have been looking for a way to combine tabletop roleplaying with grand strategy games for a long time - anecdotally, so have many others. What systems there are tend to be sets of demographics loosely and poorly plugged into existing game systems, or abstractions that basically ignore all the parts of ruling I actually want to get my teeth into - laws, trade, diplomacy, equipping and training a military, magical research and lore, domestic issues, propaganda, disasters, cultural attitudes, war.
And then this appeared, like a diamond in the rough. Straight out of /tg/ (which is 4chan's Traditional Games board, if you're not in the know) like a bat out of hell, it took the simple, stylized 'nation wars' games run there by a guy called Parrok, and refined it into a system with very few rules but an incredible amount of depth. Like Ars Magica, it's time-based, with the populace having a certain number of 'Actions' every season to perform things like building communal structures, researching new or foreign methods of doing things, making war, developing magics, in addition to the regular tasks for survival.
It simulates everything from the priest-run theocracy of sacrifice and blood to the Righteous and Glorious Republic of Rome to the local cruel lord and his put-upon peasants, even to a tiny village hidden in the woods. The only thing you need to run it is imagination and typical GM ability to present challenges that the player/nation can deal with.
It runs perfectly alongside regular roleplaying systems, in that various problems can be either dealt with directly via adventure, or the resources (Actions) of the community can be put to work to try to solve it. Characters or NPCs can be easily added to the system as individual great people are shown as an ad-hoc bonus to any action within their purview, the small bonus scale makes it easy to decide whether your party wizard gives a +1 or a +2 bonus to the Action for overseeing the magical research of the village for a season
It realizes that nearly anything is possible for a community of people to achieve, given time, and so elegantly deals with that by a very simple difficulty scaling system (measured in Acts, generated by Actions based on population size, bonuses from Great People or Applied Techs) which simply scales the difficulty up for each technique or set of ideal materials you don't have. You can totally build a road without a quarry or national history of roadbuilding techniques - you scavenge stone from fields, and figure it out on the fly, it's just harder and takes longer because there are more failures along the way.
This is the only extant copy of the rules. It's all bundled together with detritus from being run in pbp format because that's how Myth-Weavers handles archived games. There are a lot of other 'Nation Wars' games, but they have little or nothing in common with that one, because as far as I can tell Endovior wrote that from scratch and kept only the name.
Well, i'll stop tooting it's horn and give it to you to read.
Start with the Nation Building System. Take a look at the Action System. Research and Development and Technology/Technique 'Tech' Rules. Food and Gold and People: The Economy System. Randomizing Random Events: Ideas for Stuff That Happens To Your Hapless Peasants. Covert Action in Action: Rules for Ninjas. The Military System - shamelessly stolen from the World of Farland and integrated into the tech/action system.
There's more optional stuff in there, Minor Magical Disciplines, Doctrines, but the meat and drink of it is the idea of putting a community/civilization's collective resources towards achieving something, and tracking that progress via a single die roll per season towards a set number. The simple and elegant method of defining the traits of a civilization via 'specialisation', showing the management costs of large populations by giving bonuses to the die roll but not allowing more die rolls, and the underlying idea that any civilization should have challenges and disasters thrown at it on a regular basis are all just gravy, concepts that take the 'economic model' of grand strategy rpgs and hurls it directly out the window.
All in all, i've never seen anything better, and I doubt I ever will.