What would the mundane regular army of the local (human) countris be like in a fantasy world with 17th-18th century technology?

[MarkB]

The summons are a great point, and I think "what the reasons for conflict are" is a ripe opportunity for worldbuilding. Resources matter, but there is also a dynamic wherein societies with more resources engage in more conflict; see WWI, for example, taking place between the most prosperous states of the day.

So if it isn't food or whatever, is it other resources: scarce materials used as magical components, or resources which aren't more available because of magical solutions? E.g., maybe there is some inequality, with the mages taking all the land with nice views of the mountains and ocean. Or is it a response to a kind of existential ennui, a way to construct meaning in a prosperous (and boring) world? Cf Fukuyama and thymos. Is it some identitarian concerns?

OP, possibly you are aware, but it may be worth reading about the Tippyverse, an exercise in worldbuilding which took 3E RAW at face value.

When it comes to motivations for war, having actual divine beings around and involved can make religious wars more of a possibility. "My god told me to do it" carries more weight when a deity can literally turn up on the king's doorstep to tell him in person.

One wrinkle with armies marching to war in a D&D setting is that it's not exactly friendly out there in unclaimed territory. Most monsters will hesitate to attack or approach an entire army, but plenty will be happy to pick off scouts or foragers, or attack the outskirts of encampments at night.

 

[jian]

I for one would love to run Operation Night Howl past an ethics committee.

“So, the plan is that we let Sergeant Hopkins bite as many people as he wants.”

“Yes, and they’ll all turn into wereboars like him in a week or so.”

“Right, right. Are we getting signed informed consent forms from all these recruits?”

“Sure, if you like. Most of them can’t read, though, so Lieutenant Helbereth will probably have to read the agreement to them.”

“And how do we control all these lycanthropes once they’re, err, recruited?”

“Firstly they’re technically choiranthropes, not lycanthropes. And secondly, that doesn’t sound very ethical.”

 

[MarkB]

I for one would love to run Operation Night Howl past an ethics committee.

“So, the plan is that we let Sergeant Hopkins bite as many people as he wants.”

“Yes, and they’ll all turn into wereboars like him in a week or so.”

“Right, right. Are we getting signed informed consent forms from all these recruits?”

“Sure, if you like. Most of them can’t read, though, so Lieutenant Helbereth will probably have to read the agreement to them.”

“And how do we control all these lycanthropes once they’re, err, recruited?”

“Firstly they’re technically choiranthropes, not lycanthropes. And secondly, that doesn’t sound very ethical.”

Or just go with converting your existing army over to shapeshifters.

"Are you sure we're okay to do this?"

"Yes, they all signed the waivers when they enlisted."

"That waiver was about 'necessary medical procedures to maintain fighting form'."

"Yes, and biting is technically a form of injection, and as it turns out, the best form for fighting this war is pig-shaped."

 

[aco175]

When it comes to motivations for war, having actual divine beings around and involved can make religious wars more of a possibility. "My god told me to do it" carries more weight when a deity can literally turn up on the king's doorstep to tell him in person.

I smell an alignment debate.

Not that I really want to have one.

 

[jian]

Or just go with converting your existing army over to shapeshifters.

"Are you sure we're okay to do this?"

"Yes, they all signed the waivers when they enlisted."

"That waiver was about 'necessary medical procedures to maintain fighting form'."

"Yes, and biting is technically a form of injection, and as it turns out, the best form for fighting this war is pig-shaped."

It’s going to lead to some interesting monographs.

“How the Spanish Tercio Responded to Choiranthrope Formations with Specific Reference to the Battle of Oosterwheel in 1567.”

 

[Whizbang Dustyboots]

Summoned undead, summoned creatures, summoned demons, dragons, magic. Would a large army of normal men be needed? They'd just be in the way. I'm not even sure they'd be cannon fodder. I don't think large scale warfare would happen. Wars are normally fought over resources. In a magical society I don't know if those same resources would be as important.

We still use ground troops in a world with drones, nuclear warheads, submarines and whatever the heck is going on in space. Everything on your list has some pretty substantial strings attached to them. Mundane mortal troops, by contrast, are a lot easier to deal with, even if just as an occupying force.

As for the OP, I think the military would look a lot like it does in the Black Company novels: a lot of ordinary soldiers, a sprinkling of specialist spellcasters, and then a handful of incredibly powerful supernatural forces (wizards, allied magical beings, etc.) who drop in and absolutely devastate the enemy when conventional warfare doesn't work out, and then move on to the next spot.

So probably not an army of a thousand wizards, like we see in Mystara, where things get a little silly in this regard, but a handful of powerful ones, alongside a bunch of folks who can wield a wand of fireball, but aren't capable of much more than that -- and probably don't need to.

 

[GreyLord]

The other problem to think about is how common are these spellcasters.

Sure, you may have a bunch of 1st to 3rd level casters, but how many high level casters will you have. Some of those spells need higher levels, and higher level casters are not always a dime a dozen.

I think you will still need plenty of men-at-arms and Knights to go along with your spellcasters.