Tomorrow you will bring the battle to the foe. Tonight you walk among the troops you'll lead. The mercenaries are veterans and sleep while they can, but the peasants levied from your lands seek courage in tales of your previous victories. In the cover of darkness, you can listen rather than lead. Remember when you were like them? Back when you first entered the dungeon, you wielded a sword to fight for gold, glory, and the favor of the gods. Winning these earned you a throne. Now you wield an army, and fight for the right to rule the domains at war. Domains at Campaigns™ (D@W) is a supplement for Autarch's best-selling Adventure Conqueror King System™ (ACKS). Using the rules herein, your fantasy RPG characters can raise armies, campaign against enemy forces, and conquer domains. It includes a comprehensive campaign toolkit for use in ongoing games, fully compatible with ACKS's rules for mercenaries, strongholds, magic, and rulership. D@ Campaigns also includes a streamlined system for resolving the outcome of pitched battles between army-sized forces, ideal for resolving battles between NPC commanders, where the adventurers are bystanders or front-line participants. Domains at Campaigns is compatible with any fantasy RPG system that uses twenty-sided dice and shares concepts like hit points and armor class. If your players are ready to rule, then your campaign is ready for the Domains at Campaigns rules.
This supplement details armies: how to raise them, how to supply them, how to pay them, and how to move them around. It also has an abstract mass combat system (a more detailed mass combat system is in another supplement. It's kind of like splitting the Hobbit movies up when it should have just been one movie).
The problem is that you would need to be an accountant to really grasp and want to use all of this stuff. Every single thing you can do with armies comes with a math equation (and many times several math equations) that you have to figure out. Here are some straight from the book:
"Moruvai’s stronghold is situated on a hill, so the Judge consults the Assault Encounter Distance table and rolls 4d6. The result is a 20, indicating a distance of 200 yards. However, the stronghold has 80’ walls, so the encounter distance is increased by a factor of (80/6) 13.3 to 2,660 yards from the stronghold walls. The Judge uses the maximum encounter distance of 600 yards for the foray."
and
"Baal the Terrible has laid siege to Aura. Aura is an enormous city with a unit capacity of 300. Aura is a harbor city on a peninsula, and 70% of Aura’s walls touch the sea. Therefore (300 x 0.70 / 2) 105 ships are required to blockade Aura’s sea walls. 180 units (300 x 0.30 x 2) are still necessary to blockade the land walls of Aura."
It's a lot of math and a lot of work, but if you are looking for a detailed war system for your OSR games, you probably not going to get anything much more in depth. Honestly, I didn't like this book when I first read it, but upon looking over it more and more it's warming up to me. The issue is that you really do have to study this thing to really "get it".
An excellent large-scale battlesystem, useful - with slight tweakings at most - on any old-school system or retroclone. I've received feedback that the battle system itself is a bit bare-bones and gives very little for players to do - since all their tactics, stratagems, and clever moves are theroetically already taken into account with the various bonuses and penalties - reducing the whole thing to a single set of rolls even more than anything else in the system, but even so it stands a good head taller than other mass combat rules I've seen.