How would magic users be incorporated into the military?

Question

My magical world is getting into the medieval age and plate armor is becoming a thing. My magic system is simply drawing runes in the air(sometimes)/ground. A skilled person could draw a simple rune in 3 seconds. Complex runes have better range, but take longer to cast. The most a simple rune can do is accelerate an object to great speeds. Complex runes, however, can make small explosions that could take a out a small cluster of enemies.

Most people cannot do magic. In a small town 1 out of 70 people can use magic. A smaller number of people are able to draw runes in the air. What would be the structure of the military and how would these magic users be incorporated into it? What countermeasures would be used against these magic users on the battlefield?

Answer 1

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 (military wargamers) used Magic Users as a form of artillery (fire ball and lightning bolt were at-will magic User abilities in the Chainmail rules). (citation here) This was a pragmatic application of what magic can do versus what the foot and cavalry units can do. (I played a few Chainmail games before I ever knew what D&D was, and a few after).

In re psyops.

Full disclosure: I spent a career in the Military. I got to (help) write a few campaign plans, and contingency plans, for some military operations. I was formally schooled in this. Part of that professional field is in picking various kinds of units to get a particular effect or outcome - or at least to plan for that.

Magic users who can form fog clouds, gas clouds like 'cloud kill' (a variation of mustard gas) and illusions, and other super natural effects, can instill fear or doubt in the minds of opposing soldiers. That is what psyops units that I worked with IRL have been doing for decades. If you get inside the enemy's head, they are easier to defeat.

Also, in a very well known work of fiction, Sauron's whole schtick in the LoTR was a massive psyops effect - fear, corruption, evil - over most of the known world, which he wanted to conquer.

For some magic users; be part of a special ops team

A classic example of this is Poul Anderson's novel, Operation Chaos. A werewolf and a witch are a two person spec ops team who fight for one army against another.

Aerial reconnaissance is another application of magic

What would be the structure of the military and how would these magic users be incorporated into it?

Beyond the above already accepted points, those who can fly offer a huge advantage to their side by early warning and reconnaissance. It was not on a whim that planes were developed during WW I; having eyes that see farther was a huge bonus. There were hot air balloons in use in the US Civil War.
Flying scouts? Awesome eyes and ears for any battle captain!

• When the operations that I supported had a Predator on task - well, it was kind of new at the time and seemed a little bit like magic. ;-) Nowadays, they are kind of normal. About two decades ago? Nearly magical in what they allowed us to do what we could not do before.
• Twenty years before that, we had a new IR search device (FLIR) attached to our aircraft. We could see in the dark~infravision! Magic, almost. Nowadays, it's a commonplace.

Related to aerial (and other) reconnaissance: 'real time' intel.

Depending on the kind of magic that is in your world, communication to the army from various magical scouts via telepathy, clairvoyance, magic crystals, what have you, offers the field general 'real time intel' from your scout units, be they on the ground or flying. Good information on what your enemy is doing and where he is has always been valued and can be a powerful tool in deciding how to deploy your army or navy.

• Taking a tip from real life armies: the proliferation of UAVs, satellites and communications systems very much underscores how important this is. The modern professional military sorts place a high value on the "situational awareness" that comes from "all sensor fusion." Man, been some years since I used those terms, but the principles still apply. (thanks for @TheDyingOfLight for prompting me to add this!)

What countermeasures would be used against these magic users on the battlefield?

Bullets, arrows, and other magic users, as well as anything that obstructs their sight/ability to see the battlefield.

If you have not yet, I suggest to you the following work for understanding armies and the military:
Sun Tzu: The Art of War.

While written in a lyrical form, it points out almost everything you need to think about when trying to put together your army for your king/emperor/queen/whatever.

Answer 2

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 garrisons ready to call upon to defend, or a group of soldiers to move across land to occupy territory and cities, or support local authorities
• Navy: Purpose is to expand the sphere of influence of a nation around the world, act as a deterrent force (such as a fleet in being)
• Airforce: Purpose is to defend nationwide against attack, or gain the ability to quickly deal with a threat with little or no warning.

Note these do not necessarily mean ground, water and air. In fact, all three might use all elements of ground, water and air vehicles, with forces quite flexibly allocated. This is why you have marines in the Navy (ground troops on water), the Navy has its own fleet aircraft, air force it's own base and command structure. Also this is why in U.S. overseas interventions, it is the Navy's Marine Core that did a lot of the heavy lifting, because they are based in the Navy (sea-ready) and already trained for that mission.

Also, the structure enables the best assets to be developed and trained for its purpose. For instance, nation-wide defence being the Airforces it allows development of ballistic missile technology solely for that purpose.

Then, when a mission is established, it is the command structure that pulls the best fit together if needed, coordinates between forces, to accomplish its goal.

In your world, I would imagine the commanders would have at least a separate Magic User Force or Training Guild. This is so that they could collaborate and develop the best techniques regarding Magic, and also a foreign power could dedicate and train Magic Users in a similar way and therefore be vastly superior to you given development time, so you need to be competitive.

Then Magic Users would also be positioned throughout other forces depending on need and mission. For instance, you might have a small group allocated to support garrison duty, where they specialise in better fortifications. Or in expeditionary forces, where they can support logistics. Or even in standing guards, where they could specialise in subterfuge.

The best countermeasure to this approach is a similarly structured opposing array of forces, with leadership making the difference of whom is successful.

Like chess, you use your pieces best for what mission they are suited, to accomplish what main strategy you want to employ.

Answer 3

Logistics and Medical.

Assuming that your runes can be used to purify or create food and water the mages would give a logistical advantage to the army: remember that an army marches on it's stomach as Fred II of Prussia said. In naval operations they would increase the operational range of ships, both oar-powered and sail-powered.

If your runes can heal mangled man-at-arms they will give another advantage to the army by saving from death and disability those elite knights that took arrows to their kness.