Your military structure would reflect what is best for your nation to accomplish its goals
There are reasons why militaries are structured the way they are today, and historically. They all enable their primary mission focus to be best served by grouping the most effective units together.
So, very typically and somewhat broadly:
Army: Purpose is to have ...
As artillery and psyops units
Those are almost one to one parallels to the established magic user trope in high fantasy.
In re artillery
When D&D was first published, the creators were military wargamers who used Magic Users as a form of artillery (fire ball and lightning bolt were at will magic User abilities in the old Chainmail rules). (citation ...
It all depends on how magic works at a tactical level. Can powerfull AoEs be cast at a whim, or do they take some time to prepare, or do they need time to recharge? Can they be unleashed everywhere or are there restrictions? Is the location decided upon beginning the casting or when it's completed? Can it be pre-placed like mines?
Depending on this, you ...