# How powerful do I make my mages to inspire trench warfare?

The setting is somewhere near the end of the medieval era. There are knights in armor, but there are also crossbowmen shooting said knights, and mercenary armies rendering knights somewhat useless. The Black Death has come and gone, and in its wake the Christian church has begun to split. Other things are happening too, but at this point I'm beginning to forget my history.

Either way, it's an alternate history, this one with mages. Through the use of various magical materials and ancient incantations, they can provide a number of magical services, such as alchemy, healing, fortune telling, and shooting lightning bolts (from their hands, not from the sky). This last power is the one I want to focus on, as I'd like to see a medieval battlefield that closely resembles those of WW1.

See, the way I understand it, the invention of the machine gun made it impossible to charge the enemy on open terrain. Instead, everyone hunkered down in trenches and waited until mortar strikes softened up the enemy, then charged. From this, I think trench warfare could work in a medieval setting; trebuchets, ballistae, or even early cannons could act as mortars, mages as machine guns, and infantry as infantry, just without guns (but some with bows/crossbows). There may even be dragons filling in for airplanes and tanks, but that remains to be seen.

So, I guess what I'm asking is if this would work, if medieval warfare would descend into trench warfare with the introduction of mages. And if it could work, I want to know how powerful I should make these mages. On the one hand, I want them to be able to stop the enemy in their tracks, but on the other hand, I don't want them to single-handedly destroy armies. What sorts of limits could I impose on the mages to get them to this middle ground of power?

• One traditional solution is that mages spend their time in battle protecting their troops from rival mages and eliminating rival mages, rather than straight up attacking the troops. If you have an imbalance in magical power, you won't have a conflict for long. Sep 17, 2015 at 14:31
• @Frostfyre Ah, the Eragon approach. That solution always bothered me, because if you don't have a magic-user on hand, there's nothing you can do to win. I'd like to give the everyman at least a fighting chance. Sep 17, 2015 at 14:34
• These mages can shoot lightning bolts. What prevents the mages from simply shooting a couple of lightning bolts into the trench? Remember, an attacker will always (if reasoning rationally) pick the attack that gives the highest gain at the lowest cost/risk, and such an attack would seem to have a near-zero risk to the attacker and a near-certain chance of success.
– user
Sep 17, 2015 at 14:37
• The everyman doesn't stand a chance against a properly-prepared magician, though. Sep 17, 2015 at 14:38
• Note, that trench warfare was not invented in the first world war. Some battles (and especially sieges) as early as the 16th and 17th centuries did in fact involve trench warfare.
– vsz
Sep 17, 2015 at 20:22

My answer is that no you cannot, in general, make middle ages trench warfare work for the primary reason that there are not enough soldiers.

What led to trench warfare was the noted increased killing power of machine guns and artillery, PLUS the fact that both sides put enough men in the field to stretch a line of trenches from the coast of south france to the north coast of belgium.

Put even 100,000 medieval men (big army for the time) in some trenches and the enemy can just go round them and either come up form behind, or simply proceed to go and sack whatever city the trenches are supposed to protect.

On the subject of MGs versus artillery, the latter were an important part of trench warfare - since they could engage in indirect fire. You can blow up a trench with a shell but not with a lightning bolt.

So for these two reasons I think you end up with a very different dynamic, unless you have a VERY narrow frontage to the field of operations and the casting of magic bolts is a cheap as MG ammunition and you can conjure up something as generally threatening as artillery (and I don't think rock-throwers are going to qualify).

• The problem I've always had with artillery is that it seems like the best reason not to build trenches. I just don't understand why people would stop moving and dig holes when they're being shelled. Do you by any chance have an explanation of this? Sep 17, 2015 at 16:22
• @DaaaahWhoosh Earthworks give you protection against artillery rounds that miss you, but land near. Sep 17, 2015 at 16:44
• Unless the artillery shell lands right on top of the trench or dugout then the trench provides pretty effective protection but not total protection. This is why renaissance sieges featured trenches - mainly to shelter from defending cannon - only the trenches right next to the walls were within effective crossbow range. However if you do not have artillery but do have mage-proteted trenches, then the balance is tilted to much in favour of the defender since I cannot soften it up by concentrating bombardment on the point of attack. Sep 17, 2015 at 16:48
• @DaaaahWhoosh - what kills with artillery is the shrapnel and debris moving out with the blast wave. If you are standing in the open this can fly and be as lethal as bullets. In a trench, you are protected from that, and you have to be much closer to kill with concussion alone or by collapsing a well made trench. Sep 17, 2015 at 18:29
• Not exactly true. Trench warfare appeared much earlier than machineguns. There were battles, but especially sieges as far back as the 16th/17th century which involved trench warfare in at least some form.
– vsz
Sep 17, 2015 at 20:23

According to its Wikipedia page, trench warfare "occurred when a revolution in firepower was not matched by similar advances in mobility", not specifically because of the development of the machine gun. In essence, humans were developing the means to kill each other (better weaponry) faster than they were developing the means to traverse the battlefield (still using legs). From that stance alone, the scenario you proposed is believable, we just have a few things to address.

## Line of Sight

Really, everything comes down to line of sight. You can only reliably affect a target if you can see it, or there is someone who can relay directions to you, such as with artillery fire. If you want to limit your mages, you need to prevent them from being able to see into the opposing trenches and state a rule they can only interact with things they can see.

## Power

What's the last place you want to be in a flash flood? If your answer is "a trench" you're on the right track. Trench warfare is the opposite of what you want to do if your opponent can raise the tide, make it rain, or simply redirect a river. Not that there's really anything to be done if your opponent can do those things... Anyway, you want to limit the singular power of your mages to ensure the clever one in the bunch doesn't, for example, drop a guided boulder from a phenomenal height.

## Endurance

A mage's staying power in combat is the final piece of the puzzle. If one mage can hurl a lightning bolt every five seconds all day long, the attacker is never going to get anywhere without severe casualties. To inspire trench warfare, you need to not place definitive limits on your mages, such as the as-much-energy-as-your-body-can-sustain tactic from Eragon or the X/day casting limit common in Dungeons and Dragons.

• You could also require that the magic is constrained by the law of conservation of energy.
– user
Sep 17, 2015 at 16:42
• @MichaelKjörling There's always another solution. Thanks for the link. Sep 17, 2015 at 16:45
• Indeed; just that I recalled that we had discussed that particular limitation here. You're welcome.
– user
Sep 17, 2015 at 16:49
• Very good. I was about to cite wikipedia's entry as well. I think that is the key for @DaaaahWhoosh. You need to have plenty of firepower, but no speeding. So no dragons. And of course strong limits on the mage: limited use per day and limited variety of spells. As you mention, otherwise no good defensive tactics could be used. One idea could be that mages cannot fight themselves on the field, but can produce magic missile wands with limited number of uses, and usable by other people. Sep 17, 2015 at 20:13
• ok, I made it an answer on its own. Sep 17, 2015 at 20:42

Like an army equipped with firearms and artillery, your mage would need these things to do that:

• Recharge time: The mages need to be able to cast fast enough. Long incantations means that the enemy has enough time to close the distance with the mage or to take cover midway. Muskets were slow to recharge and the battles ended in a grand mêlée. Your mages do not want that because they are going to die, unless other soldiers can protect them.

• Distance: Being able to shoot far away is a great advantage. The enemy is less likely to be able to close in.

• Accuracy: Shooting at a long distance is useless without the proper accuracy. the combination of the two makes it deadly for soldiers to stand in an open field. They will try to take cover. Mage can use area attacks, so accuracy is less important maybe.

• Power: Early weapons were weak and this affected all the other characteristics badly. Being able to fire large mortars on the enemy efficiently is much more destructive that using simple bullets.

• Endurance: And lastly, you need your mages to be able to cast enough spells to be able to push back all possible attacks at any time. Individually, mages would need to be able to fire a lot of spells per day. But you also need to have a lot of mages to make rotations or to serve as back up if the other get out of spells/energy.

The mages must be able to fire spell with a combination of the previously mentioned qualities if they want to keep the enemy at bay. Trench warfare is a war of attrition not a war of maneuvers. The two army prefer to stay behind their fortifications because it's their only way to minimize losses.

I think the best way to achieve this scenario would be to have mages with a very high number of daily spells, moderately powerful, low accuracy making them almost useless at long distance but deadly at close range. That's the thing, you just need to keep the other army at a distance. And protect your mages with the infantry just in case.

• I think it would make sense to depart from the "DnD model" of mages here and remove the spells/day limit altogether. Also, an alternative to low accuracy is to make spell power fall off with distance, or make recharge time increase with distance. Personally I'd go with the latter since it gives you the possibility of a long-range but low fire rate 'sniper mage.' Sep 17, 2015 at 16:16
• @2012rcampion: Another possibility is to increase the cost with distance; it need not even be linear, falling off with the square of the distance would make it extremely expensive to attack at long range. Sep 17, 2015 at 17:25
• "Vancian magic" is the term for pre-prepared spells and limited casting per day, based on the works of Jack Vance. Sep 17, 2015 at 20:17
• @2012rcampion It has nothing to do with Dnd. Unless the casting time of a spell is 0 and they require no energy (or no special effort of any sort). , there is a limit of spell you can cast per day. My assumption is that spell require some time to cast, at least 1 second or so and that each spell will drain some physical or mental energy from the user (similar to mana). Eventually, he might need to take a break: it's like he's trying to sprint a 42km marathon. Sep 19, 2015 at 2:45

So, I guess what I'm asking is if this would work, if medieval warfare would descend into trench warfare with the introduction of mages.

That depends on what, specifically, the mages can do. They can throw lightning... but can they conjure up shield spells to repel lightning? If so, then you have a lot less of a need to hide in trenches. Can they teleport? Can they teleport a group of soldiers along with them, or even without them? If so, there's no need at all to hide in trenches. Can they use their power to make moving around large amounts of earth a simple task? If so, then you can bring whole new levels of strategy to trench warfare now that the trenches are no longer static. (Especially if they're covered over the top with shield spells!)

And if you're going to get into writing a story about what mages can do, I would strongly recommend you have a look at a series of articles by the master of that particular topic, Brandon Sanderson:

Sanderson's First Law, in which he explains how and why the ability of an author to use magic to solve problems (without the audience feeling like it's a cheap trick on the author's part) is directly proportional to how much and how well the author explains what the magic is able to do.

Sanderson's Second Law, explaining why limitations on powers are more interesting to the audience than powers, and more useful for building drama.

Sanderson's Third Law, which recommends that, rather than introduce something new out of the blue when you need to expand your magic, it's better to build on what you already have. (And three ways to do it well.)

Read those three and think about what your mages can and can't do, and you'll have a much easier time establishing your world of (maybe!) magical trench warfare.

• Sanderson's 3 laws of magic are gold! Definitely take a look at them. Sep 17, 2015 at 20:17

First we need to analyse what factors made WWI into trench warfare. Those factors are: rifles, machine-guns, artillery, grenades, transportation and the size of the conflict.

Then, we can think about how your magic can emulate those factors. Finally, I'll touch on how those aspects effect staples of the fantasy setting: knights, close combat, and medieval army structures.

CREATION OF TRENCH WARFARE

How it happened historically; and how your magicians can do it

Rifles

The rifles of WWI are a factor in creating trench warfare, because they had much greater range, accuracy, and rate of fire compared to firearms of conflicts beforehand.

Rifles were intended to be shot up to 1.5 to 2km away to saturate an area with bullets and deny it to the enemy. A lot of rifle historians (e.g. Paul Scarlata) consider this not very effective as a tactic, but either way most rifles from 1870 to WWI were built with sights intended for this very long range. Whether it worked well or not, it was expected, and people were trying it.

So, your mages should have long-range rapid-fire saturation effects - such as your mentioned lightning bolts.

Rifles varied of course in accuracy but are capable of immensely precise shots, more so than a crossbow. There is a lot of crossover here with range as a factor. Armies up to the 16th century could and would line up within a kilometer or within a mile without getting shot to bits. A crossbow is accurate enough to aim at a single person and hit half the time maybe at 50-100 yards. You can aim at a group of men at 200-300 yards. That's it, and the longbow and all firearms up to about 1550 have comparable range and accuracy. Muskets up to about 1840 would only max at about 400 yards. A rifle of the first world war has a much better accuracy. Off-hand I can think of this example of long range shooting (http://www.youtube.com/watch?v=fhaY3pa1oio). That's hitting a 3-foot by 10 inch target half the time at 1,000 yards. It's also an older 1886 rifle without smokeless powder or spitzer bullets, so is inferior to many of the 'new' rifles of WWI. But will serve as an example.

A modern charger or box magazine rifle of WWI could shoot about 20 times per minute. Those magazines were introduced 1886-95 in most nations. but many still had older rifles in service with reserve troops etc. France had 150,000 Gras rifles (1874), and 10% of Russia's rifles were Berdan IIs (1870), but these could still do 10 shots a minute or more. So if your mages are going to replicate a rifle, they need long range and a high rate of fire.

So, your mages - if you want to emulate a rifle - need to be able to threaten groups and areas 1500 to 2000 yards away, need to be able to hit a man 1000 yards away, and shoot 10-20 times a minute. That's not to say that to create trench warfare you actually need to emulate a rifle, or use these exact figures, but they are an option.

Machine Guns

I will say now it was not machine-guns that created trench warfare. In 1914 and 1915 machine-guns were generally available at about 2-4 per regiment (1000+) of infantry. Of course their presence was powerful, but in a small area. A strategic location. Put low-grade mages throughout your army to represent rifles, and have individual or small groups of exceptionally powerful casters in strategic locations, like how machineguns were used at first.

As the war progressed, more and more machineguns until they were about 10 times more common (2-4 per company of 100-200 troops), and special assault units had even more, had light machine guns etc. Having your mages emulate late-war machineguns means employing them as strike teams - actually similar to how tanks were then used.

Artillery

Artillery was a far bigger killer and wounding weapon than machineguns and even more than rifles, even though every soldier carried one. I would say rifles created the trench warfare situation and artillery just made trench warfare a lot more bloody, but it could be argued better artillery forced more trenches and earthworks.

Quick bit of background. Most artillery until about 1860 was smoothbore and muzzleloader. Many nations got guns which were either rifled (much longer range) or breech-loading (much higher fire rate) around 1860, and get the two combined about 1870.

Then the 1897 French Gun comes in (https://en.wikipedia.org/wiki/Canon_de_75_mod%C3%A8le_1897). Being recoilless, it massively improves the accuracy and rate of fire possible for the crew. Every nation quickly gets some of this gun (e.g. UK and USA) or makes something similar (e.g. Germany's 7.7 cm FK 96 n.A) and that's the standard artillery of WWI - WWI was also the first really major usage of such a brutally efficient machine.

I have already stated my historical opinion ("rifles created the trench warfare situation and artillery just made trench warfare a lot more bloody"), but one could say that 1897 type of artillery is what made WWI so different: artillery's immense range and indirect fire with newly improved accuracy and rate of fire.

The equivalent would be a Dungeons And Dragons style spellcaster with an extended range Fireball at will that they could shoot outside of line of sight.

WWI was the first major usage of really reliable grenades. They were mostly fielded by dedicated assault teams, similar to Light Machine Guns, in an attempt to break the trench warfare. Not really a factor in creating that style of static warfare. It's easy to picture a mage with short range attacks (such as D&D's 'Burning Hands' or 'Circle of Death'). Do remember that grenades are more indirect than traditional short range fantasy magic, though.

Overall in weaponry, you can create trench warfare by making spellcasters dominate the field at long range, forcing everyone to hunker down in trenches. If you want to keep crossbows etc. relevant, then maybe drop some of the range for spellcasters down to 400 or 500 yards or something. Enough to make close combat less viable (thus trenches) but also enough to keep them special compared to mundane weapons. More on close combat further below.

Transportation

The first mass use of railways for warfare was the 1870 Franco-Prussian War. Prussia prepared a war plan, tricked France into declaring war, then immediately mobilised their massive army and shiny new artillery right into strategic points of France before France had had time to prepare. France had a much better rifle (1866 Chassepot vs the Dreyse), had experimental machineguns (the Millatreuse), but lost spectacularly, mostly because the Prussians caught them by surprise. Which they did by mobilising effectively: getting their troops and their big Krupp artillery into the best spots.

It's not hard to imagine magic portals and gate spells working similarly, although teleporting a whole army (or even one mage) anywhere in the world is not going to work. You're going to have to limit it somehow: e.g. you can teleport X people per day. Or better, you can only teleport from one portal/standing stone/magic circle/ancient ruin, to the next (nearest) portal in the chain/line. Like how a railway train runs linearly.

Air travel can be emulated too. WWI had a small number of scouting planes and (ineffective compared to WWII) bombers. Mages can very easily replicate this, or characters riding giant eagles or similar.

Mining and sapping under strongpoints was also common late in WWI. Easy to create a magic tunnel, or even teleporting a squad at fairly short range.

Size of The Conflict

As the first world war took place across whole borders, nay across the whole of Europe, with millions of troops, whole countries were fortified. It was a frontier. There were few places where a strike force could slip in to a country unnoticed.

Trench warfare did occasionally happen in the middle ages, but only in sieges or while defending a town etc. (see FR Taylor's "The Art Of War In Italy 1494-1529" for some nice examples, even if that's later than the medieval you want to style the world on).

In a medieval war, you have pockets of 1000 and 10,000 men garrisoned in strategic points or wandering the land living off pillage, maybe a total of 100,000 men in your army in total. Armies often bypassed each other, flanking and outmaneuvering were the order of the day. Your 'frontier' was a few castles and towns in an otherwise massive empty stretch of land. There are some exceptions though, e.g. where your southern border is a river. This is where your (smaller population) fantasy world can work.

For your world-building, borders are best placed on a river or ridge or ravine or some other natural feature (e.g. NOT like the USA's mostly artificial borders between perfectly square states). Fantasy worlds are more likely to have epic mountain ranges, grand canyons, raging rivers and other dramatic geographic elements suitable for a frontier. 5000 men guarding a long stretch of open plains is hard, but 5000 guarding a torrentious river with only a few bridges is much easier to picture trenches and static fighting around, interrupted by some specialist assault teams with magic and armour (more on this below).

HOW TRENCH WARFARE AFFECTS THE FANTASY WORLD

And how you can keep your traditional fantasy elements

Knights (Part 1, Armour)

Armour other than basic helmets was only really worn in two years of WWI: 1914 and 1918, the first and the last. Some cavalry were still wearing cuirasses and breastplates during 1914, before the trenches really stuck in. Later in the war, tank crew, machinegun crews, and special assault teams were given breastplate-type armour. No nation could afford armour for everyone (and it was hot and heavy so not that popular), so they assigned it as strategic points: just like how machineguns and tanks were.

Your high level mages (being machineguns and tanks) should be focused in strategic points to attack, and so could your armoured knights. I am picturing the trenches being held mostly by crossbowmen with leather armour and kettle helmets, with a few low-level spellcasters to act as snipers; but attacking strategic points, a few special assault groups of high-level mages and armoured knights.

Knights (Part 2, Cavalry)

Cavalry and Trench Warfare don't mix very well. After 1914, once the trenches were established, cavalry on the western front were mostly kept in reserve, or would be reassigned left and right to different parts of the line as reinforcement. Then they dismounted and fought with rifles. This would work fine for light cavalry, but isn't very knightly. Knights charge in, right?

Close Combat

Now, close combat did happen in the trenches. The first version is a suicidal bayonet charge (or your crossbowmen's daggers, axes and clubs) across no-man's-land. But this didn't happen very often in reality.

The second is a precision strike by specific assault teams. I mentioned earlier how both high-level spellcasters and armoured knights can fill in that assault role. In WWI, the armoured people would be carrying grenades and light machine guns.

In your world, you can separate them into unarmoured mages carrying fireballs, and armoured knights carrying swords and shields. Half a dozen of each would be an assault team which could take on a strongpoint. If you want a medieval strongpoint emulating those of WWI (bunker and MG), then have a dugout with a Wall of Force or Wall of Fire (the bunker) staffed by mages (machinegun).

Army Construction

The army structure and raising methods of WWI is very different from the "feudal system" in fantasy worlds (which didn't really exist in real society - see Melissa Snell's "The F Word: Feudalism" for a summary). However, it's not hard to think of a way they could be combined.

The King demands an army, and assigns a particular area/border/province of the frontier to each lord. Each Lord brings their elite knight-retainers, pays some mercenaries, picks half of each town guard, and levies some peasants. Each Lord also has a court wizard or two who comes along, and can bribe or blackmail one of the local colleges of magic to provide some mages.

So, each lord acts as a lieutenant-general or similar rank. Levied peasants, town guards and mercenaries with crosswbows form the front line of the first trenches, and take massive casualties from enemy mages. Meanwhile, the lord sends the weaker mages to reinforce the trenches ala machine guns and snipe. Some moderately powerful mages are kept as reserves and/or as bombardment artillery.

Then the lord sends his dozen best knights and powerful court wizard with 6 talented apprentices on a specific mission to assault the enemy strongpoint in the mountain pass.

• Awesome detailled answer ! It deserves more upvotes ! I really like your insight on trench warfare ! Jun 30, 2020 at 14:40

### Trench Warfare

Like Frostfyre's answer, I think that the key point is starting to understand the basis for Trench Warfare. The Wikipedia article mentions that

Trench warfare occurred when a revolution in fire-power was not matched by similar advances in mobility, resulting in a gruelling form of warfare in which the defender held the advantage.

So you need to design your mages to do that. Considering the medieval armies: horse mounted, infantry (pikes, cross-bows and bows), you need to get your mage to favour considerably the defence compared to the attacks, even on an open battlefield.

### Wanted effect

When two similarly equipped armies with similar numbers, the presence of mages should make it such that your cavalry does not want to go charging. Which makes it pretty clear that your mages should provide offensive spells with a limited range but a large area. If in one go, you get 5-10 knights down, and you can go on, such that when the knights arrive in contact with your infantry they are so weak that they lost all the advantage of the charge; then you're good. You are mentioning lightning bolts. In some RPGs, there are area-based shower of lightning bolts. So you get less precise, but more targets at once.

With such a spell, the other armies probably would prefer to wait for you to come at them.

### One possible implementation

But now you need to get some limitations to your mages, to explain why you don't simply send a squadron of them forward. And also why they aren't using other spells to destroy the trenches: instant transport in there and start firing, flood, fly over, etc. As indicated above, their fire-power should surpass their mobility.

One such possibility is that your mages are not on the battle field, but work in magical factories to produce wands of lightning. Those are produced charged with a limited number of spells (25-50) and can be used with specially trained but non-magical units, which are on the front. Since the training is costly, you don't want to send those units ahead and charge the enemies, but you'd rather have them take care of the protection.

Why do you want to do it that way? Maybe because the magic only allows indirect actions. Maybe incantations require so much time that it would be impracticable on the battlefield. Maybe it takes so much time to study magic, that all your mages are too old to go to the field.

• Harry Turtledove's Darkness series handles this idea very well....well, at least the "what happens after you have a magico-industrial complex" Mar 13, 2017 at 19:22

Its not necessarily one component but several. I have thought of this lately as well. Things that I have considered.

Trench warfare occurred when a revolution in firepower was not matched by similar advances in mobility, resulting in a grueling form of warfare in which the defender held the advantage. In World War I, both sides constructed elaborate trench and dugout systems opposing each other along a front, protected from assault by barbed wire.

First the basic Soldier Crossbows are a must. Perhaps repeating crossbows. Knights are pretty much done for as the whole reason trench warfare started is due to lack of mobility. With all the craters and hazards in no mans land it would be extremely difficult to maintain cohesion for an effective cavalry charge.

Artillery The main killer. You have the right idea. Trebuchets, catapults, Batista and lots and lots of them. They can lob boulders, flaming pots with nails, perhaps some sort of alchemist gasses, greek fire, etc...

Machine Guns Your "machine guns" could be The Scorpio-Ballista (see roman) where they fired serial bolts.

Airplanes The dragons could be airplanes but seem a bit overpowered, perhaps Pegasus archers (think mongols but on Pegasus.)

It seems like you have most of this down and I only point this out to show that you can have conditions much like WW1 WITHOUT the need for mages.

Now onto your question. To answer this question more fully I would need a better idea of what your magic system entails. Is this D&D magic system?

If you insist on having mages then IMO any power level could be appropriate. As pointed out above you can already get very close WW1 conditions. You need to be careful with the mages becoming so powerful that a lone mage can single handily wipe out and entire defended positions simply by sitting in a bunker and lobbing fireballs over and over. Also if you have teleport you can now have a "paratrooper" style troop where the mage acts as the transport and can teleport in elite troops. Manipulation of the weather could be used as well.

Could you clarify more exactly what your magic entails?

• I can't really clarify, as I was hoping to build the magic based on the needs of the setting. I think your answer is pretty much complete as it stands, at least for my purposes. Sep 17, 2015 at 15:30
• Cool. The only other consideration I just thought of would be that the mages are like Spec Ops. Perhaps teams of 2-4 for special elite missions. Sep 17, 2015 at 15:42
• The dragons could be airplanes but seem a bit overpowered. That depends on what dragons can do in-universe. (They tend to be all over the place in fantasy literature.) If you can find it, you may want to check out Christopher Rowley's Bazil Broketail series, about military life where dragons and humans fight side by side as regular troops. Sep 17, 2015 at 19:00

In brief, make the mages as powerful as WW1 machine guns (maybe a bit more) while avoiding the introduction of rapid mobility or mobile armor.

If the mage's rate of fire isn't as good as a WW1 machine gun, then the slow rate of fire can be compensated for by making every lightning bolt count.

Can you get from open battlefields to WW1 trenches with the introduction of mages that shoot lightning from their fingertips? Yes, but probably not for long.

If no increase in mobility or conventional armor matches an increase in killing power, then enemy troops will have to adapt in novel ways. The adaptation in WW1 was to make dirt into armor. Lots of dirt is lots of armor. However, the problem with dirt is that it's hard to move and impossible to wear. Thus the stagnation of trench warfare.

By WW2, the Germans (and soon everyone else) developed smaller, faster, sufficiently armored tanks that could withstand machine gun fire without damage. The increase in mobility by both sides eliminated the need or opportunity to build trenches.

Historical Context

Since WW1, we've seen an incredible increase in guided precision munitions. Firing thousands of rounds of artillery shells or machine gun bullets are less necessary when a single well aimed shell or bullet will suffice. The machine gun was an industrial age answer to the problem of how to put more bullets in the air. Single shot or semi-automatic weapons were unable to put down the suppressing fire required to counter massed troop formations.

Mages with lightning bolts coming from their fingers don't have that problem, since every shot goes where it's supposed to.

Machine gun fire by itself did not create the conditions for trench warfare. Siege lines from as far back as the late middle ages resembled Great War trench lines since the attackers were essentially forced to advance against a superior defender (protected by walls, having greater height to shoot from and so on). Photographic evidence from the American Civil War also demonstrates the use of trenches and other fortifications, despite both sides primarily armed with rifled muskets and smoothbore cannon as field artillery.

As noted in several answers, the main issue is the fire is not matched by mobility. Anything which enhances fire or restricts mobility is going to lead to a situation where "trench warfare" type conditions evolve, and this can include accurate and rapid fire weapons (magical or ordinary), artillery which outranges the field force weapons (it wasn't until about the Russio-Japanese war that artillery commonly outranged the enemy rifles) and field fortifications like barbed wire. You could invert your backstory by having magical field fortifications like quicksand or walls of thorn bushes that prevent rapid movement across the field, rather than lightning bolts, for example.