Traditional historical battles didn't have wizards that could cast cloudkill.
If you want to write a guide for DMs that want to have wars in their campaigns in low/no magic games, or traditional wars that they justify somehow (The wizard's guild kills any spellcaster who meddles in politics from their flying doom sky fortress) (The kingdom of Amnar exists in an antimagic field and also has all the gold - let's take that fvcker!) that's totally okay, but you don't get to say that the existence of full-casters doesn't obsolete historical warfare.
A standard DnD war is a lot more like a modern battlefield. Assuming of course NPC unoptimized casters and not optimized ones, who break the idea of 'war' in half completely.
'Air Support' in the terms of flying monsters or casters or high level rapidshot splitting spellstoring archers on flying mounts swoops above the battlefield. A hidden MG nest on a clifftop opens up on a file of foot soldiers, three wizard spitting dozens of magic missiles per round out of wands at the spread out footsoldiers. 'Artillery' in terms of fireballs from half a mile away, or even any of the magical ballistas like fire, ice, or necromantic. Use of the Water To Acid spell to create acidic projectiles for catapults. Far Shot + Greatbow + spellstoring arrow (fireball) is like a V2 rocket. Basic low level wizards or sorcerers using 'guns' (wands of magic missile) to snipe at each other from opposing trenchworks. Magical means of conveyance are more like railroads than wagon-chains. Large summoned monsters or buffed fighters/gishes are the Tanks, bashing aside terrain obstacles, crushing enemies beneath them, and having a caster and some apprentices ride them to act as their main gun and sponson/axial machinegun, as well as to refresh buffs or even heal them.
With some other less common options, even common soldiers could use spell-traps of magic missile to shoot at each other, and a soldier with a burning hands on a stick becomes Flamethrower-Guy.
Summoned monsters, machine gun nests, landmines, trench warfare, magic guns, buffed tanks (make it a buffed warforged to get iron kingdoms Warjacks), vast BFC effects (hallucinatory terrain, silent image, Move Earth, wall of fire), gas (cloudkill), artillery (the humble fireball, others), air forces (creatures with wings, mages, flying mounts)...
Regular non-conscripts with spell turrets soldiers would fill the role of 'commandos' more than anything. Vast numbers of conscripts WWI style in trenches with spell turrets (magic missile, or searing ray for heavy weapons troopers) to supplement the armour and the air force, supported by the artillery. Professional soldiers become the sargeants of the trenchworks, leading the conscripts to victory/death, or work together in small groups to assassinate enemy casters or otherwise monkeywrench enemy plans.
Really, War in DnD is amazing. The kind of different tactics all the options magic gives could create vastly different forces. Unfortunately the various 'heroes of battle' etc books assume magic is used up close and personal and everyone hits each other with axes. And the only different tactics that are used is that dwarves are defensive and have axes and drow skirmish and use bows and light blades and poison.
The arch-artificers of Mnoyl and their Construct Legions vs the Mid-Royanaran League and their professional and technologically equipped army - constructs flying through the air fighting the warmages of the third Aeromancy Group, the metal constructs of the Ironworkers Guild advancing on the ranks of the Royanaran heavy foot, Large tower shields giving full cover against long-range fireballs while the magic missiles of the shell-shaped guns of the Royanarans deflect off the tough outer armour of the constructs. Summoning circles behind the Royanaran line spit forth a long contingent of Fire Creature Fire Giant Mercenaries from the plane of Fire, who use their burning attacks to melt through the construct's armour, while taking casualties from the claws of the constructs and their electrified armour. Rayemi berserkers from the Rayemi mountains of Royanar, hired for the occasion, charge forth from behind the lines, Huge sized, with Abjuration Guild wizards on their backs, warding them against harm and 33rd Wartransmuter Regiment casters making them unstoppable with buffs.
That is how war goes in DnD. Historical warfare needs a) not to have warriors that can kill whole legions of troops on their own (heracles, beowulf, level 10 fighters) and b) not to have magic. Pretty much any magic makes 'traditional' warfare pretty versimilitude breaking. Cause it doesn't make sense that over the course of campaign history no-one would have figured out that wizards can kill massed formations of troops pretty easily. Spreading out those troops then leads to an arms race that ends with far fewer traditional troops and many more wizards, trained animals, monsters, and things like spell-turrets.
