Specific relations with settlements and sovereign powers, or even regions of land, can be determined in the world building files devoted to those. Large forces like an army are comprised of people throughout a power, but they’ll have military bases in specific communities. This will be strategic locations and often large population centers, so if we’ve created a map or otherwise decided where major settlements are, this decision can be made for us.
Smaller forces like a knighthood may exist anywhere there’s a need, including in or near smaller settlements. It is these that we may be to decide on a case-by-case basis for which communities have them in quantities beyond the lone person. Assess the threats posed by animals, monsters, and species found in each land feature (such as mountains and forests) near a settlement; armed forces are designed to protect against such threats.
Creating Places (The Art of World Building, #2) taught about the different varieties of terrain: open land, forests, hills, mountains, deserts, and swamps/jungles. Few military groups operate equally well over these terrains, and not at all in some. This helps us decide how commonly encountered they are. Terrain also impacts their transportation choices. For your files, state what types of terrain they’re found in or what sort of encumbrance they experience if traveling there, such as being slowed down or having to go around.
For example, horsemen would be stopped by a jungle, but a savannah would be more like open land; the difference is underbrush. We don’t need to note how they fare in every forest type. Instead, state that underbrush and low branches slow them and that the latter can also impede the use of certain weapons like the sword, leading them to use shorter blades. We may need to remind ourselves of their reduced effectiveness in certain conditions so we can more realistically portray and use them. They may become known for one, which may have led to their development. Horsemen excel at open land, whereas a force that rides dragons or large birds might specialize in mountains because they can fly over the terrain that hampers others.
Multiple species help us because each may be helped or hindered by different terrain. Imagine our armed forces reaching a jungle and stopping because the humans on their horses can’t continue, which prompts a recruitment effort seeking out members of a species which can. This is unlikely to be a secret, leading to open acknowledgement of the need and role these species fill. Maybe every squadron of these horseman is expected or required to have them.
These terrain decisions help us determine where the armed forces are found. A city surrounded by jungle won’t have a cavalry at all, but one with open plains in every direction certainly will. A mixed-terrain settlement will utilize the most appropriate group based on circumstance. This can also lead the population to think more highly of one group than another, namely the one seen as more responsible for protecting them. That can cause tensions and resentment among rival military groups. All of this adds believability and layering to our setting. It’s easy to create a character who belongs to one group and has an attitude about anyone from the other group. The amount of each territory within their jurisdiction aids the decision.