Well, we can all debate the merits of different fantasy tabletop wargaming systems, but this is the kind of "How would the Battle of Fredericksburg have gone if Joe Hooker had Iron Man armor?" question that D&D was basically designed to answer.
I mean the closest thing I can give to what I consider a "realistic" answer is that it looks exactly the same as warfare in the Napoleonic Wars in the first place; except the 10% of the population who can use magic are the ones giving the orders. The people getting hurt are going to be the same motley collection of hard-bitten half-orc mercenaries, gruff but lovable dwarven gunnery sergeants, and idealistic farm kids off to seek their fortune as always.
I mean, your first difference is right there: Ask yourself what your society considers even cheaper than a human life and that's what's on the front lines. Necromancy in this context would be an area worth exploring. Maybe there are only so many soldiers at all at this point and they just keep getting resurrected and death warded and revivified over and over. And they're all 20th level by now and questioning whether they couldn't do better for themselves.
Battlefield healing magic has eliminated infection, but typhoid and dysentery still sweep through the camps, because there just aren't enough slots for that and clerics consider physicians to be leech-applying superstitious primitives who aren't willing to follow the science and become priests.
The next consideration is there are only a dozen spells with a range of over 500 feet, while Napoleonic rifles were good to 200 yards and artillery, obviously...
If the world in which you want to build your novel conforms closely to D&D rules (and I'd personally reconsider that), the one area where I think you'd see a LOT of real game changing variation would be something like artillery. This ties into a thread from a few weeks ago: the leader who uses his nation's meager supply of adamantine making an armor suit for himself is exchanging a moment of vanity for a future of futility. The ones who use it to perfect a breech-loading artillery mechanism quantum-jump from the Napoleonic Era to Civil War/Crimean War Era or even later in terms of their ability to rain metal on people far away.
If you want to go in the non-technological direction, a wand of magic missiles is cheap and dirty, instantly lethal to a 1st level character, and cannot miss. If you can get a bunch of them within 120 feet of the enemy position, you're golden. Maybe high-magic low-science countries try to use human wave tactics to close to effective range. They'd be perfect for night attacks where nobody can see past 60 feet.
Magic would be the heart and soul of communications, intelligence, counter-intelligence; counter-counter-intelligence; diplomacy... It would also be the ultimate terror weapon. But in a world where it's more valuable than gunpowder, gunpowder will drive it out of the marketplace.
ETA: Engineering, obvs, would be another big one. The ability to manufacture cover instantly ON the battlefield would be huge.
Haven't read all the posts, so maybe I am repeating something already mentioned but IMHO, using a high level spellcaster by one side would be considered similarly as nuking other country in the real world, and would probably end in total destruction of all involved parties.
Well, we can all debate the merits of different fantasy tabletop wargaming systems, but this is the kind of "How would the Battle of Fredericksburg have gone if Joe Hooker had Iron Man armor?" question that D&D was basically designed to answer.
I mean the closest thing I can give to what I consider a "realistic" answer is that it looks exactly the same as warfare in the Napoleonic Wars in the first place; except the 10% of the population who can use magic are the ones giving the orders. The people getting hurt are going to be the same motley collection of hard-bitten half-orc mercenaries, gruff but lovable dwarven gunnery sergeants, and idealistic farm kids off to seek their fortune as always.
I mean, your first difference is right there: Ask yourself what your society considers even cheaper than a human life and that's what's on the front lines. Necromancy in this context would be an area worth exploring. Maybe there are only so many soldiers at all at this point and they just keep getting resurrected and death warded and revivified over and over. And they're all 20th level by now and questioning whether they couldn't do better for themselves.
Battlefield healing magic has eliminated infection, but typhoid and dysentery still sweep through the camps, because there just aren't enough slots for that and clerics consider physicians to be leech-applying superstitious primitives who aren't willing to follow the science and become priests.
The next consideration is there are only a dozen spells with a range of over 500 feet, while Napoleonic rifles were good to 200 yards and artillery, obviously...
If the world in which you want to build your novel conforms closely to D&D rules (and I'd personally reconsider that), the one area where I think you'd see a LOT of real game changing variation would be something like artillery. This ties into a thread from a few weeks ago: the leader who uses his nation's meager supply of adamantine making an armor suit for himself is exchanging a moment of vanity for a future of futility. The ones who use it to perfect a breech-loading artillery mechanism quantum-jump from the Napoleonic Era to Civil War/Crimean War Era or even later in terms of their ability to rain metal on people far away.
If you want to go in the non-technological direction, a wand of magic missiles is cheap and dirty, instantly lethal to a 1st level character, and cannot miss. If you can get a bunch of them within 120 feet of the enemy position, you're golden. Maybe high-magic low-science countries try to use human wave tactics to close to effective range. They'd be perfect for night attacks where nobody can see past 60 feet.
Magic would be the heart and soul of communications, intelligence, counter-intelligence; counter-counter-intelligence; diplomacy... It would also be the ultimate terror weapon. But in a world where it's more valuable than gunpowder, gunpowder will drive it out of the marketplace.
ETA: Engineering, obvs, would be another big one. The ability to manufacture cover instantly ON the battlefield would be huge.
Battle mages would replace heavy artillery, military monks could literally be dropped from airships, the possibilities are endless.
Mystic v3 should be official, nuff said.
Haven't read all the posts, so maybe I am repeating something already mentioned but IMHO, using a high level spellcaster by one side would be considered similarly as nuking other country in the real world, and would probably end in total destruction of all involved parties.
(OP seems to want to scale back "high level" magic)
I'd think that offensive magic, like fireball and whatnot, would look more like close air support.