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As you might expect, a military campaign involves a lot of mass combats. Although mysteries, dungeon crawls, and wilderness adventures all come up occasionally, mass combats will be the plurality, if not the majority, of a military campaign. As such, this is a campaign type for people who like to fight major wars and build large empires almost exclusively. For those who like world domination but also want to be closer to the action and/or roleplay more often (not to say that military campaigns are devoid of roleplay, but it does become more rare as the amount of combat skyrockets), a sphere of influence campaign might work better.
A mass combat game divides the world up into a few dozen territories. Each territory has an army attached to it, which is usually at least six or seven individual units. Big enough that you could fight a mass combat with it if you had to, but small enough that you’d rather not. Some territories, densely populated and wealthy ones, might support a much bigger number of units in their army. For example, Waterdeep has a huge population and plenty of money, so their army holds something like twenty units. On the other hand, some territories are sparsely populated and contain relatively few units in their army. Icewind Dale, for example, might have as few as four units in their army. Each army can do basically two things: It can move, or it can replenish itself.
A military campaign’s turns are each one season long, and most units can only move and fight during spring, summer, and autumn. Some settings might have a different set of seasons, for example, they might have three dry seasons (which in-character would just be referred to as one dry season, but it’s three times as long as a temperate season) and one rainy season, and depending on how heavy the rains are, standard units may or may not be able to move and fight during the rainy season. For (usually) three seasons out of the year the party can push their armies forward and conquer new lands, and then in winter there’s a chance for courtly intrigue or some good old fashioned dungeon crawling before spring comes and the armies march again.
Winter is also a perfect opportunity to replenish the lost forces of an army. An army must have a supply line connecting it to its home territory to replenish and if it replenishes it cannot move or attack, which means if an army replenishes during another season, it will get left behind. An army that is replenished gets back all of its destroyed units, but you can only put the squeeze on one territory so many times before it stops producing any new forces. The second time you try to replenish the same army, you can only replenish up to half the units in that army (which can still bring it up to full strength if it was at one half or more strength to begin with), and the third time you replenish an army, you get only a quarter of the units for that army. An army can only be replenished three times. After that, there’s simply no more bodies that territory can spare.
Some, perhaps even most, of the territories on the map will be more or less neutral. Perhaps not neutral in the sense that they may in fact be full of hostile raiders who are basically at war with you most or all of the time, but neutral in the sense that they aren’t fighting total war until you bring an army to their doorstep. These enemies defend their territory with their army, and once the party has captured the territory, that army will flee into neighboring territory if they have an more territories or allied territories adjacent to retreat into or, if they don’t, disband. The party must then use the full refresh of the territory to raise the army again, which means except for their starting territory and any territories brought in diplomatically, they must use the first and only full refresh of a territory just to get an army out of that territory at all.
Some enemies might expand their territories much more voraciously than others, being actual rival nations with their own expanding empires. There are two ways to approach the fights between these rival empires and small, neutral nations. Players (but not usually their characters) can take control of the neutral nations’ armies for the mass combats, fighting perhaps against overwhelming odds to try and do as much damage to the enemy army as possible, softening them up for when they confront the enemy army with their own forces. Alternatively, you can compare forces, make some rough estimates about how many of the enemy empire units would be defeated, roll the INT saves to see if those units are annihilated or not, and get back to the game.
Either way, enemy empires should be common and aggressive. If there’s only one (which is usually wise if this is the first military campaign you’ve run), it should have territories fairly close to the party’s starting territory, so that the party is fairly quickly fighting an enemy who can field armies just as large as they can. Once the party has a few territories under their belt, the army they can bring to bear against a neutral territory will be massive enough as to be a foregone conclusion. The main role of neutral nations past the first few battles of the campaign is to tie down some of the party’s forces. A neutral territory next to a completely undefended territory, or just one lightly defended enough that the neutral territory is confident in victory, might take advantage and capture it, immediately cutting off one of the party’s armies from being replenished (you can’t have a supply line running from the home territory to the army with no hostile forces intervening if the home territory is itself a hostile territory). Even worse, some neutral territories might capture a territory that cuts off a mountain pass or blockades a port or otherwise controls some chokepoint, squeezing off half a dozen of the party’s armies from reinforcements. As the party advances, they will need to either dedicate time and forces to clearing out low value territories or else leave behind armies to keep the inhabitants of those low value territories from getting any ideas.
On a map with multiple enemy empires, as the party gets more powerful, the enemy empires should begin banding together to stop them. This naturally escalates the scale of the conflict so that the party begins as one territory fighting other one-territory neutral enemies, then fights a single enemy empire with four or five territories once they have four or five themselves, and after conquering that empire they’re confronted by a pair of neighboring empires who between them have similar territory to the party’s now sizable empire, and so on. If you want to have only one enemy empire but you’re insistent on making the map very large, devise some reason why the enemy empire won’t bring all of their forces to bear on the party’s growing forces immediately. Maybe there’s a friendly empire on the other side of the enemy keeping the bulk of the enemy forces busy, but being slowly overwhelmed by a more powerful foe.
Since each territory produces an army with an average of about six units, you don’t want the total number of territories to be larger than about three dozen. Even with that number, you want about half the territory to be neutral and make sure enemy empires keep the party’s attention so that most of that neutral territory remains neutral rather than the party gobbling it all up before attacking their larger rivals. With 36 territories, a battle between the party empire and an equally large rival (or coalition of rivals) who have divided the world between them would have 18 territories for an average of 108 units a piece. While you want the final battle to be larger than the standard mass combat, and while some units will have been annihilated and be unable to be replaced during the build up, a fight with anything even close to a hundred units per side will see each side ordering a comically small number of their units into battle each turn. Keep the pressure up and make sure that the players end up picking and choosing which neutral nations they want to invade. If that sounds like a balancing act you can’t manage, stick to smaller maps. A twelve territory map will lead to a final battle of 36 units to a side if evenly matched. That’s a very large battle, but not completely unmanageable.