GURPS Mass Combat is a tool so that you can run a fun and exciting War Campaign.
I've trying to run a war campaign for the longest time. I've made several attempts and there are just too many problems at times. I do admit that I can have a demanding set of players who are all military history buffs, which is actually a good thing in my opinion.
The problem with war campaigns is that everything about war is rooted in the real world. Why certain tactics and techniques became dominant and obsolete reflects how reality shaped the "game" of war. The everything about the past shaped the resources, strengths, actions and technology used in war. Adding on or take away aspects wrecks havoc in the same way as removing an aspect of nature would damage the ecosystem.
This economics and ecology made War Campaigns very difficult and at best, frustrating to the people who run it. The funny thing about the situation is that History Buffs are the ones who want to play it, and History Buffs are the ones most likely to be frustrated by it.
Right now I am at a point to have a very good Idea of how a War Campaign is supposed to go. Much of the problems I faced, way back when I was running them and attempted to run them again are being answered only recently.
When I ran them before, I was overwhelmed by the options and tactics my players would use to achieve their goal. A war campaign that felt real is much like a sand box setting, too many options, that it overwhelms the GM and not the players. The mil-his players know what they are going to do, they only need to work with the information they are given and the realistic constraints.
The GM, on the other hand, has to simulate the entire environment and an adapting enemy who is ignorant of the PCs own decisions. He has to know almost all possible consequences and enough realistic constraints to make a decent facsimile of a real war. He then gets pretty overwhelmed at this point. Worse is the Players may not have much of a challenge, because the GM has to boil down a lot of things to keep things running smoothly.
It doesn't help that, there is no single source to look at when trying to run a medieval/ancient war machine. There is also no single tool by which to look at war. Limitations on budget, time and opportunity tends to tie down the GM to a point of paralysis.
Right now, I've pretty much figured out how battles are run. I have to communicate that template to my players and to anyone who cares to listen. I must have it in writing or else I'll forget it myself. The template is not there to constrain them, but for them to know the language of the opponent's actions. Without this "language" or template, no one will know what the other is doing or trying to hide.
Breaking down the decision making process of the commander requires a look at how that he understands the organism of his own estates. He knows the results of higher taxes and how it actually kills growth. He knows that in war troops desert and die and his troop numbers will steadily fall in the course of the campaign. He also knows what would motivate men to throw their lives at his command.
War in itself is a hot bed of all the things that make up humanity in that era. Everything observable about humanity can be found in their greatest and most decisive conflicts. No one just jumps into war, even if few of the warriors and the king might relish the idea. Everyone is actually generally scared for their lives. Picking out the details and inductively developing a robust hypotheses of the era's psychology is the best way of going about understanding how it is to make war but requires constant improvement.
Anyway, I've come up with a list of things I have to do to prepare for a war campaign. One of the easier things to do is to download some historical maps of the eras I'd like to play and print them out in a 4x3 ft map tarp.
Then I have to consider carefully which "era" I can most effectively build on for a game. I've made some Mass Combat Cards, because I discovered the best adventuring organization for a party is at a company level.
The company level is explained here. In the Strategikon and History of Wars, the most basic unit is the Band, Cohort, Tagma, or Company (they are all the same thing). In the History of Wars by Procopius, it is this unit that is sent out to do raids, recon and trouble. They are the same size of units being sent by the enemies. Every leader of a company is special enough to be named in history.
In medieval organization, the Companies or Cohorts have a special place in Germanic Culture. You will find this when epics and poems (like Beowulf). These are a collection of exalted men, men who are leaders in their own right but are bound together in a powerful fraternity. You will only get this idea after lectures of Germanic culture after the fall of Rome.
The focus on the scale of 200-400 fighting men is a matter of population, economics, travel, logistics, and casualties. The best of these men, the handful of key different individuals, 4-12, that holds a company together makes it even more perfect for the purpose of highlight the variety that can be found in an adventure.
It is from this roster, players can choose where they can specialize. When some of the PCs advance, they might break off and form their own Bands. Eventually they can form a battalion. The level of organizational flexibility is pretty high and sustainable at this level. Any larger requires a grasp of military operations and any lower is not sufficient to become a notable threat to a population of a town or city.