Some thoughts by BOS:
Intelligence: Rogue & ranger scouts, invisible wizards assisting. Alarm spells for early warning. Diviners casting divinations to search out enemy battle plans. Wizards scrying areas to look for troop concentrations. Teleporting scouts appear & disappear behind enemy lines. Counter-intelligence/reconnaissance with spells like undetectable location, forbiddance, alarm, etc. Druids in animal form make stealthy, long-duration surveillance units.
Battle Command: Spell-casters of all types using sending, or wands of sending, or animal messengers to relay messages. Telepathic magic to link members of a unit so they can communicate by thought. Scrying plus illusions allow battle tracking in real-time inside command posts across a broad battlefield (coordinated by sendings & other communication). Most magical communication can't be intercepted, too!
Air Defense: Attack spells plus long-range archers and other missile fire do a number on flying creatures. Walls of force help to protect from aerial attack.
Fire Support: Spellcasters & boom spells. Spellcasters on flying mounts ... or with dragons, spellcasting flying creatures. Plenty of ways to put firepower anywhere you want on the battlefield.
Mobility/Countermobility/Survivability: Increase mobility with travel magic (teleport, etc) to bypass obstacles, spells to re-shape obstacles (passwall, etc). Emplace quick obstacles magically, whether by wall of iron/force/etc, cloudkill. Fire spells, entangles, webs, etc help canalize enemies. Digging creatures help shape the battlefield. Various magics help to both protect and attack by causing natural elements to change.
Maneuver: Travel magics, elementals, and flying creatures open up the third dimension and enable the vertical envelopment. It creates a fluid, mobile fight with no front or rear. Heavily armored creatures like bulettes, augmented by spells, lead breakthroughs of prepared defenses. Stealthy strike teams, or teleported assassins, augmented by magic take out high value targets. Buff/scry/teleport tactics mean no commander is safe without deception measures and spell-based defenses. Summoned creatures open new fronts and allow attackers to hit their enemies from multiple directions simultaneously.
Logistics: With create food & water, an army no longer marches on its stomach. Bags of holding and portable holes make moving large quantities of supplies child's play. Spells can make tireless mounts, or reduce loads for the approach march with things like floating disk, or reduce. Planar travel (esp astral) moves massive armies long distances without the need for supplies.