How would a magical war go? Like, what would be the set up for platoons specifically?
How magical, and what kind of magic?
The problem with a question like this is that, from a world building perspective, magic basically takes the form of alternate technology. It exists and develops alongside technological advancement. In some cases it may evolve faster, but, the overall concept is identical.
So there’s two factors we need to look at. How powerful is your magic, and, how common is it?
The more powerful your magic is, the more it will disrupt how warfare is fought. This is also true for overall political power in your world. The more powerful your mages are, the more they’ll be able to completely exclude non-mages from all power structures.
For example, if your mages can casually obliterate non-magical infantry, your world won’t have much use for “conventional,” infantry. You might even see mages waging war against one another directly via spellcraft, rather than any conventional concept of warfare.
Why invade a resisting city when you can rain fire on it, or consume the souls of all it’s non magical residents turning them into a kind of zombie? Why not just drop it into the sea, and be done with it?
This is where the exact nature of magic in your world becomes very important. You need to create rules for how magic works and then plan accordingly.
How common magic is in your world also heavily influences warfare. If magic is incredibly rare. If mages only come from a few noble bloodlines, you’re not going to see a lot of magic on the battlefield.
On the other end of the spectrum, if magic is relatively common. If anyone can be taught to cast basic spells, you could easily see a situation where combat magic is the norm. Where every soldier in a battalion was expected to understand a basic ranged spell, and a shield against incoming spells.
Worth noting, when I’m talking about how common magic is, there’s a few potential factors to consider. How often can someone cast? Are there any significant costs associated with magic? How hard is it to teach? And of course, who can cast magic? Obviously, if your setting allows anyone with some education to cast magic, that’s going to look very different from a world where magic is exclusively the purview of a few hereditary bloodlines.
If magic is powerful enough, but it takes some time to train up magic users, you might see a situation where military forces constricted significantly. Where a few squads would be considered enough to secure and occupy an entire city.
Similarly, if spells have a considerable cost associated with them, or can only be cast on very restricted schedules, that will have less of an overall impact on the way your world develops than if they can cast at will.
Another important question is, “what are they fighting for?” Historically, more wars have been fought over resources than ideas. When your world allows for basic transmutation of one good into another, for example converting something into gold, then gold has no value. You can’t fight over gold, because it has no value. If a mage can conjure up enchanted plate, then steel isn’t going to have much market value. If a mage can easily produce enough food to feed a thousand, then you won’t have a need for farmers or agriculture. Things get weird. Do empires war over magical materials that are consumed to produce goods? Do they battle over nexuses of magical energy? If they can use portals to bounce around at will, do they even bother securing their own borders, or do they operate out of heavily fortified enclaves leaving everyone on the outside to fend for themselves?
As a writer building a world, magic is open to your imagination. You can do, nearly, anything you want. The only thing you’re tested on is how creative can you be? Can you create a scenario that fits the shape of the world you want? Magic in warfare can be anything from magical artillery to squads of superheroes. The only question is, “what do you want to do?”