They would never deploy a force to any possibly strategically significant location that could be wiped out by a group of 6 low level (and out of 20 levels ... level 6 is still low) characters.
Well....
I am not entirely sure about this statement. Yes there is a range of 20 levels, but if you plotted the distribution of character levels in the world, you wouldn't get a uniform distribution, nor even a Gaussian (peak in the middle at level 10-11). Rather, what you'd have is something more like a an exponential decay -- most of the observations would occur at the low end (low level) and progressively fewer would occur as you go up in level. In other words, most of the world doesn't even have a class or level (i.e., "level 0"), and then the majority of "leveled" characters would be level 1, with half as many level 2s, half again as many level 3s, and so forth. Following this progression, a level 6 party would be in something like the 97th percentile of the world (top 3% of all leveled characters). Just because there are 14 more levels of characters above them doesn't mean that the world is equally distributed.
You can see this among monsters as well. If you search DDB for CR 1 to CR 1 (i.e., only CR 1 monsters), there are 8 pages of results. If you search for CR 10 there are only 3 pages of results. CR 15, 2 pages. CR 19? 1 page. Furthermore, many lower CR creatures come in packs, tribes, or herds, whereas the higher CR creatures tend to come as singles. Add to that, many high-CR creatures don't even come from the normal material world, but are extra-dimensional (demons, celestials, devils, elementals). Depending on the world, these may not be the sorts of things an army would ordinarily expect to fight.
In other words, the world is skewed toward lower level (and CR) beings, so an army wouldn't necessarily expect or need to prep for attacks by very high level beings, because there aren't that many of them, at least not in the material plane.
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Also, all the combatants out there KNOW that there are roving bands of adventurers (unless in your world the party is a unique group of heroes). They would never deploy a force to any possibly strategically significant location that could be wiped out by a group of 6 low level (and out of 20 levels ... level 6 is still low) characters.
Depends a lot on how strategically significant it is. The reality is, there's generally more stuff you want to protect than you have forces to significantly garrison. If it's in the top ten significant targets, it's going to have more forces than a party of level 6 adventurers can handle. If it's in the top hundred, that's a maybe. If it's in the top thousand, probably a party of adventurers won't have a significant problem on an initial attack, though the response may be overwhelming.
I did a quick skim, but didn’t see this specifically mentioned:
Suply Lines. The bulk of an army’s resources are actually spent on logistics. How to get enough food, shoes, medicine, uniforms, ammunition, and other vital equipment from the staging areas behind the lines, all the way up to the front for the people who need that stuff. The invading force would likely have at least one non-combatant for every armed combatant. Blacksmiths and farriers, and fletchers, cooks, ostlers, etc. etc.
Small, fast units like scout companies or light cavalry could scavenge, hunt, and steal what they need for themselves. An army will quickly deplete the available resources of an are in relatively little time. So... where are the supplies?
Supplies would depend on what kind of support the army has. It's an army so it must have some sort of patron. That being said, a couple clerics or druids to make food and water and a couple wizards for mending and done deal.
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Supplies would depend on what kind of support the army has. It's an army so it must have some sort of patron. That being said, a couple clerics or druids to make food and water and a couple wizards for mending and done deal.
Yeah, the exact number of spellcasters available matters a lot. A single Goodberry is food for ten, a single Create or Destroy Water is water for ten, so one level 1 druid or level 2 ranger per 10 soldiers will do the job. If we assume adversary forces in modules are typical, though. the normal ratio of spellcasters to soldiers is lower than that.
Supplies would depend on what kind of support the army has. It's an army so it must have some sort of patron. That being said, a couple clerics or druids to make food and water and a couple wizards for mending and done deal.
But consider Biowizard's (correct) post about the scarcity of people with class levels. If you have clerics, druids and wizards at your disposal, even if they are level 1, you are going to have much more effective uses for them and their spells slots then feeding the grunts. More likely you'd maintain mundane supply lines for the typical soldiers and send the folks with class levels on special forces type missions.
I might suggest that you divide all those troop numbers by 10? One, mass combat isn't D&D's strength. Two, nothing short than a city is going to have enough food for 100 extra men, let alone 1000, let alone 10000. Especially not before the advent of the Agricultural Revolutuion. No need to send out Chinese Empire numbers of troops. Which brings the question, how did this invader keep the gate fed and supplied? Clearly controlling the border between their enemy and a third country is going to be more difficult than controlling the battleline. One would assume the supply line would be emperiled.
You have to be careful here, D&D is not the medieval times when a local lord could at best maintain a few me-at-arms because of food supply. D*D is about epic battles, and few people in LotR wonder how armies of tens of thousands are grown and maintained to clash at Minas Tirith. D&D is also about magic and again, the Cyclopaedia goes into great detail about the effect of spells like Create Food and Water. Or, in the Stormlight Archives, there is soulcasting to maintain the huge armies on the Shattered Plains.
D&D is vaguely medieval, so maybe reduce the armies to 3-5k soldiers, which means maybe this gate-- seemingly a low value strategic target, unless they are expecting the enemy is going to receive reinforcements-- might only have 10-25 men. Remember, post-Roman Europe didn't reach armies of like 100k until the late 18th century. So much of society was required to commit their labour to farming that large militaries was mostly impractical-- if they picked up spears or muskets, then no-one was growing the food to feed them.
You are right, it is only "vaguely" medieval, it's also extremely magical, and this needs to be taken into account, and this is why you need to adjust for at least medium to heavy magic, whether on the battlefield or in supply lines. The first, although not simple, is a bit easier because you can consider magic as artillery and higher level characters as tanks or aircraft, but that also gives you ideas about the fact that a phalanx, while being probably one of the most efficient forces on a antique/medieval battlefield because of troop density is probably not that good in a medieval setting. Napoleonic era moved to squares for defensive positions, but even then they did not have that many explosive shells on the battlefield. And then you can go to the US and see how war evolved when "magic" became of a higher order and how formations evolved.
It's harder for supply and not as interesting in a sense, but there are many ways to supply troops with magic as well, so you might as well assume that they are being done. That being said, amongst the things that the adventurers could do to weaken the force, assassinating the chaplain might be even more worthwhile than assassinating the Battle Mage. :)
I'd argue D&D is not about epic battles. Maybe D&D is about battles like Moria, but certainly not about battles like Helm's Deep. It's, at best, about dozens of people, not about hundred or thousands of them.. That's why there are not army battle rules in the system.
Honestly, massed armies don't make sense in D&D; it's far too easy to wipe out massed troops with AoEs. You might have them anyway, because cinematic trope (just like you might have medieval-ish castles, which also don't make sense), but the 'realistic' conflicts would probably be more like late 19th/early 20th century warfare, when artillery and machine guns were kings of the battlefield.
Supplies would depend on what kind of support the army has. It's an army so it must have some sort of patron. That being said, a couple clerics or druids to make food and water and a couple wizards for mending and done deal.
But consider Biowizard's (correct) post about the scarcity of people with class levels. If you have clerics, druids and wizards at your disposal, even if they are level 1, you are going to have much more effective uses for them and their spells slots then feeding the grunts. More likely you'd maintain mundane supply lines for the typical soldiers and send the folks with class levels on special forces type missions.
If the army is in the game with PCs involved, then the army WILL have casters of some sort. Having an army of 0 level shlubs when PCs are involved is stupid.
During anything except combat, having casters feed the army is important by orders of magnitude.
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"real life is a super high CR."
-OboeLauren
"............anybody got any potatoes? We could drop a potato in each hole an' see which ones get viciously mauled by horrible monsters?"
See here, but I only partially agree. First, it depends on the strength of the magic in your world. In LotR, magic is not a major factor in warfare, whereas it is in the Malazan Book of the Fallen. But even in that case, high level magic tends to neutralise itself as powerful mages are also concerned about their own skin and vulnerabilities, and unless a side has a tremendous advantage in that respect (a bit like air power advantage in the real world), it does not necessarily prevent mass tactics.
It's very difficult to prevent a fifth level mage from either disguising himself as normal military or turning invisible, dropping a Fireball or two, and running away before he can really get ganked (various options can be taken to increase safety, . Even worse when he's level 7 and uses Dimension Door to run away. Sure, there's a chance of a Counterspell, but only if he's recognized as a threat before explosions start going off, and there's a possible opponent within range.
Against massed infantry, that's going to result in 20+ kills. That might be acceptable if you're guaranteed to actually kill the mage, but you aren't; you have a chance, but it's probably something like 10%, and a 200:1 trade just isn't sustainable. The obvious answer is that you use dispersed formations where you don't get 20-30 kills from a fireball, you get 3-5.
Honestly, massed armies don't make sense in D&D; it's far too easy to wipe out massed troops with AoEs. You might have them anyway, because cinematic trope (just like you might have medieval-ish castles, which also don't make sense), but the 'realistic' conflicts would probably be more like late 19th/early 20th century warfare, when artillery and machine guns were kings of the battlefield.
I'm not sure how what you're describing as Civil War to WW I era battles takes away the idea of "massed armies." Both wars involved using massed armies, as well as smaller wars in between.
But quite apart from the battlefield we know that without a massed army, you can't take and hold possession of a large area. That is, a massed army might not guarantee you could hold an area if there are high level mages about. But without some sort of massed army, you absolutely could not hold it, because it is the strength of numbers that holds large areas. This is why even today we might send in small units of special forces to soften up an area, but when you want to take and hold the area you send in the infantry. Navy SEALS might be able to sneak into an area and take down its command/control and even nail its leadership, but the SEALS cannot hold a city or a countryside by themselves. You use the 1st Infantry division for that.
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The proportion of leveled characters and higher CR NPCs in a D&D world is DM/world dependent.
If the party is one of 3 to 5 similar adventuring parties throughout the entire world then it might not be strongly considered as a possible issue when deploying troops. Leveled characters would be just so uncommon it isn't worth considering.
However, out of a population of 1,000,000 ... if just 10% of the population (1/10) has some capabilities it gives at least 100,000 individuals with class levels or equivalent. This could means a minimum 10,000 individuals into tier 2 and tier 3 (if only 1% of the population achieves higher levels or equivalent). If an army is 35,000 ... you could potentially have it composed of entirely leveled characters or CR1/2+ creatures since the army may represent better prepared fighters with training (leaving aside untrained conscripts or levies). Note: for comparison, the population of medieval Europe was about 50 million and the Roman Empire was 50 to 70 million people.
So, when I DM a D&D world, small groups of comandoes composed of elite individuals would certainly be a consideration when deploying troops. Other DMs would have other opinions and their worlds may differ.
However, just looking at the monster manual section on NPCs (and of course a DM can use whatever they want).
Berzerkers CR2 - "Hailing from uncivilized lands, unpredictable berserkers come together in war parties and seek conflict wherever they can find it."
One might reasonably expect entire warbands of Berzerkers to be a possible threat.
On the military side you have:
Guards (CR 1/8) "Guards include members of a city watch, sentries in a citadel or fortified town, and the bodyguards of merchants and nobles."
Knights (CR 3) "Knights are warriors who pledge service to rulers, religious orders, and noble causes. A knight's alignment determines the extent to which a pledge is honored. Whether undertaking a quest or patrolling a realm: a knight often travels with an entourage that includes squires and hirelings who are commoners"
Scout (CR 1/2) "Scouts a re skilled hunters and trackers who offer their services for a fee. Most hunt wild game, but a few work as bounty hunters, serve as guides, or provide military reconnaissance."
Veteran (CR 3) "Veterans are professional fighters that take up arms for pay or to protect something they believe in or value. Their ranks include soldiers retired from long service and warriors who never served anyone but themselves."
The MM could really use a CR1 or CR1/2 military type NPC like a solider. Untrained conscripts could be represented by commoners. In any case, veterans represent experience soldiers and would likely be NCOs and officers. More challenging military ranks could be filled using the Gladiator NPC. Knights would also have officer roles. You could have exceptional units composed mostly of veterans but those might be uncommon or rare. Depending on the context, these NPCs could be a reasonable choice for putting together a military unit. Each squad of 10 could have 7 guards, a scout and two veterans for an experienced and trained unit. Organize the army from squads to companies to brigades ... add in support in the form of mages, priests and acolytes with the occasional druid or two tacked onto the recon and support company or squads. With an army organization in mind it becomes easier to figure out what forces might be assigned to what tasks ... then just let the party face whatever is sent. You don't want the players to get the idea that the world revolves around them :)
I'm not sure how what you're describing as Civil War to WW I era battles takes away the idea of "massed armies." Both wars involved using massed armies, as well as smaller wars in between.
I was specifically referring to the dense troop formations you see in a lot of cinematic battles, and which were historically used at times (e.g. the Greek phalanx, Roman infantry tactics, pike and halberd formations). The battle of Helm's Deep in the movies had the orcs using dense multi-line spear formations (basically equivalent to pike formations), and those are just meat for attack spells.
I'm not sure how what you're describing as Civil War to WW I era battles takes away the idea of "massed armies." Both wars involved using massed armies, as well as smaller wars in between.
I was specifically referring to the dense troop formations you see in a lot of cinematic battles, and which were historically used at times (e.g. the Greek phalanx, Roman infantry tactics, pike and halberd formations). The battle of Helm's Deep in the movies had the orcs using dense multi-line spear formations (basically equivalent to pike formations), and those are just meat for attack spells.
*Warning this is extremely euro-centric because that's the history I know*
There isn't a sensible distinction to be made between Classical/Medieval massed formations and Napoleonic War/American Civil War (and to an extent WW1) massed formations in the context of this discussion. An infantry square is a tercio with no pikes, a tercio is a schiltron with added arquebus/muskets, a schiltron is a phalanx that happens to be in the medieval period. The specific weapons being used differ, but we have now almost 2,500 years of recorded history in which people have fought using an evolution of essentially the same tactics.
All these formations were used on battlefields that had weapons that were capable of dealing with massed troop formations, so why did massed troop formations stick around for so long?
Because you were still a lot safer, and more effective as part of a massed formation than as an individual. Change out a battery of field guns for a squad of wizards if you like, but I don't see any fundamental difference in how a cohort/company/insert other organisational term here of infantry are going to deal with that - they are either going nowhere near them and calling in skirmishers or cavalry or charging them down to negate their advantage.
As to your example of the mage using magic shenanigans to run away - at that point he haslost. He might have killed 30-40 people, but ultimately he has retreated because the enemy have attacked him with a force he cannot withstand.
There isn't a sensible distinction to be made between Classical/Medieval massed formations and Napoleonic War/American Civil War (and to an extent WW1) massed formations in the context of this discussion.
The sensible distinction is that they were largely ineffective and a tactical mistake as of the late 19th century. Now, people do sometimes persist with obsolete ideas for a while in the face of new developments, but all evidence is that broad area spells are not new developments in typical D&D settings.
As to your example of the mage using magic shenanigans to run away - at that point he haslost. He might have killed 30-40 people, but ultimately he has retreated because the enemy have attacked him with a force he cannot withstand.
First of all, the fact that the mage has retreated does not mean his supporting troops have retreated. Secondly, it's only a loss to retreat if there's some particular value to holding your current position, but there are plenty of skirmishes where your primary objective is destroying enemy forces. If your goal is holding ground you probably use a different spell mix.
First of all, the fact that the mage has retreated does not mean his supporting troops have retreated. Secondly, it's only a loss to retreat if there's some particular value to holding your current position, but there are plenty of skirmishes where your primary objective is destroying enemy forces. If your goal is holding ground you probably use a different spell mix.
A mage facing a large force and only able to destroy 20 at a time is going to be at it for a lot of days, or even weeks, to have any real effect on a big army. By then the enemy force might have already captured its objective or overrun it.
As a general rule, killing lots of enemies is inferior to taking objectives, which is why Patton did so well in WW II vs the less effective generals like Montgomery, who were trying to "wear down" the Germans. Yes, Patton's army did achieve a higher kill rate, but he also out-flanked, surrounded, and out-raced the German army to multiple objectives by using his army's mobility (which is why he achieved the superior kill rate). I don't think Patton would have cared a whit if some grenade-chucking enemy (i.e., the mage) were plinking at his army while he was capturing a city. This is exemplified by my uncle, who fought in Patton's army and quoted him as having said, "I don't care how many dog tags you have to bring back, I want that hill." A direct quote from the person who is considered one of the most effective, if not the most effective, generals in the history of warfare.
The mage who is using hit and run tactics could have made Patton's army bleed a little, but hit and run never took a hill (and more importantly, held it).
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The sensible distinction is that they were largely ineffective and a tactical mistake as of the late 19th century. Now, people do sometimes persist with obsolete ideas for a while in the face of new developments, but all evidence is that broad area spells are not new developments in typical D&D settings.
Really? Things were certainly beginning to swing away from massed formations by the late 19th century but you cannot seriously make an argument that infantry armed with, say, chassepot breech-loaders and supported by mitrailleuse are capable of the sort of squad-based maneuver warfare that would become the standard by the Second World War - one troop of cavalry comes their way and they are all toast. Conflicts in this period were exceptionally bloody, but we need to remember the context that these were literally the largest armies that history had ever seen - none of this should take away from the absolute horror and pointlessness of these conflicts, but how else are you planning to fight these wars before motorisation and the light machine gun. The Prussians during this period certainly began to innovate maneuver warfare, but the main reason it was successful was the sheer number of people they could put into the field.
First of all, the fact that the mage has retreated does not mean his supporting troops have retreated. Secondly, it's only a loss to retreat if there's some particular value to holding your current position, but there are plenty of skirmishes where your primary objective is destroying enemy forces. If your goal is holding ground you probably use a different spell mix.
I was assuming that these hypothetical troops were commanded by someone competent and wouldn't decide to charge randomly at anyone they happened to see. If there is no tactical advantage to the position the mage is holding, why would they even bother to approach? Why is the mage even there? Also, the main objective of skirmishes is almost never simply to kill people - the main objective is usually securing or screening the deployment of the main force. Like BioWizard said above, a good commander is concerned with objectives - both strategic and tactical - and assuming the mage isn't a complete moron and standing in the middle of a random unrelated field, we just made them move off the objective they were holding.
Mages are something akin to artillery. Warfare has had cannons, catapults, and other forms of artillery for a long time. Or perhaps not even that ...
A modern day hand grenade has a thrown range on the order of 100' and a lethal radius on the order of 16'. This is remarkably similar to a fireball with a range of 150' and 20' radius.
Hand grenades are ubiquitous in modern warfare. Many troops might have one. A level 5 mage is almost the equivalent of a soldier with 2 hand grenades to throw. Is their impact on a force really that devastating from an actual damage point of view?
Anyway, in order to get within fireball range, that mage is within 150' of their target. Longbows have a normal range of 150' and long range of 600'. If a mage reveals themselves by casting a fireball, then they will become an immediate target for every archer within range. Having a strong long bow force then becomes a clear counter measure to enemy mages though having some of your own mages would also be a good idea.
It can be interesting discussing the effects of magic on how warfare would develop in a D&D world ... but most of the 5e D&D combat rules are aimed at small scale skirmishes and not at larger scale warfare. However, one of the big changes between 5e and earlier editions is that previously a high level character might be able to take on an entire army and expect to win while in 5e a level 20 character might be defeated or at least forced to flee by 100 low level NPCs.
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Well....
I am not entirely sure about this statement. Yes there is a range of 20 levels, but if you plotted the distribution of character levels in the world, you wouldn't get a uniform distribution, nor even a Gaussian (peak in the middle at level 10-11). Rather, what you'd have is something more like a an exponential decay -- most of the observations would occur at the low end (low level) and progressively fewer would occur as you go up in level. In other words, most of the world doesn't even have a class or level (i.e., "level 0"), and then the majority of "leveled" characters would be level 1, with half as many level 2s, half again as many level 3s, and so forth. Following this progression, a level 6 party would be in something like the 97th percentile of the world (top 3% of all leveled characters). Just because there are 14 more levels of characters above them doesn't mean that the world is equally distributed.
You can see this among monsters as well. If you search DDB for CR 1 to CR 1 (i.e., only CR 1 monsters), there are 8 pages of results. If you search for CR 10 there are only 3 pages of results. CR 15, 2 pages. CR 19? 1 page. Furthermore, many lower CR creatures come in packs, tribes, or herds, whereas the higher CR creatures tend to come as singles. Add to that, many high-CR creatures don't even come from the normal material world, but are extra-dimensional (demons, celestials, devils, elementals). Depending on the world, these may not be the sorts of things an army would ordinarily expect to fight.
In other words, the world is skewed toward lower level (and CR) beings, so an army wouldn't necessarily expect or need to prep for attacks by very high level beings, because there aren't that many of them, at least not in the material plane.
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Depends a lot on how strategically significant it is. The reality is, there's generally more stuff you want to protect than you have forces to significantly garrison. If it's in the top ten significant targets, it's going to have more forces than a party of level 6 adventurers can handle. If it's in the top hundred, that's a maybe. If it's in the top thousand, probably a party of adventurers won't have a significant problem on an initial attack, though the response may be overwhelming.
I did a quick skim, but didn’t see this specifically mentioned:
Suply Lines. The bulk of an army’s resources are actually spent on logistics. How to get enough food, shoes, medicine, uniforms, ammunition, and other vital equipment from the staging areas behind the lines, all the way up to the front for the people who need that stuff. The invading force would likely have at least one non-combatant for every armed combatant. Blacksmiths and farriers, and fletchers, cooks, ostlers, etc. etc.
Small, fast units like scout companies or light cavalry could scavenge, hunt, and steal what they need for themselves. An army will quickly deplete the available resources of an are in relatively little time. So... where are the supplies?
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Supplies would depend on what kind of support the army has. It's an army so it must have some sort of patron. That being said, a couple clerics or druids to make food and water and a couple wizards for mending and done deal.
"Sooner or later, your Players are going to smash your railroad into a sandbox."
-Vedexent
"real life is a super high CR."
-OboeLauren
"............anybody got any potatoes? We could drop a potato in each hole an' see which ones get viciously mauled by horrible monsters?"
-Ilyara Thundertale
Yeah, the exact number of spellcasters available matters a lot. A single Goodberry is food for ten, a single Create or Destroy Water is water for ten, so one level 1 druid or level 2 ranger per 10 soldiers will do the job. If we assume adversary forces in modules are typical, though. the normal ratio of spellcasters to soldiers is lower than that.
But consider Biowizard's (correct) post about the scarcity of people with class levels. If you have clerics, druids and wizards at your disposal, even if they are level 1, you are going to have much more effective uses for them and their spells slots then feeding the grunts. More likely you'd maintain mundane supply lines for the typical soldiers and send the folks with class levels on special forces type missions.
I'd argue D&D is not about epic battles. Maybe D&D is about battles like Moria, but certainly not about battles like Helm's Deep. It's, at best, about dozens of people, not about hundred or thousands of them.. That's why there are not army battle rules in the system.
Honestly, massed armies don't make sense in D&D; it's far too easy to wipe out massed troops with AoEs. You might have them anyway, because cinematic trope (just like you might have medieval-ish castles, which also don't make sense), but the 'realistic' conflicts would probably be more like late 19th/early 20th century warfare, when artillery and machine guns were kings of the battlefield.
If the army is in the game with PCs involved, then the army WILL have casters of some sort. Having an army of 0 level shlubs when PCs are involved is stupid.
During anything except combat, having casters feed the army is important by orders of magnitude.
"Sooner or later, your Players are going to smash your railroad into a sandbox."
-Vedexent
"real life is a super high CR."
-OboeLauren
"............anybody got any potatoes? We could drop a potato in each hole an' see which ones get viciously mauled by horrible monsters?"
-Ilyara Thundertale
It's very difficult to prevent a fifth level mage from either disguising himself as normal military or turning invisible, dropping a Fireball or two, and running away before he can really get ganked (various options can be taken to increase safety, . Even worse when he's level 7 and uses Dimension Door to run away. Sure, there's a chance of a Counterspell, but only if he's recognized as a threat before explosions start going off, and there's a possible opponent within range.
Against massed infantry, that's going to result in 20+ kills. That might be acceptable if you're guaranteed to actually kill the mage, but you aren't; you have a chance, but it's probably something like 10%, and a 200:1 trade just isn't sustainable. The obvious answer is that you use dispersed formations where you don't get 20-30 kills from a fireball, you get 3-5.
I'm not sure how what you're describing as Civil War to WW I era battles takes away the idea of "massed armies." Both wars involved using massed armies, as well as smaller wars in between.
But quite apart from the battlefield we know that without a massed army, you can't take and hold possession of a large area. That is, a massed army might not guarantee you could hold an area if there are high level mages about. But without some sort of massed army, you absolutely could not hold it, because it is the strength of numbers that holds large areas. This is why even today we might send in small units of special forces to soften up an area, but when you want to take and hold the area you send in the infantry. Navy SEALS might be able to sneak into an area and take down its command/control and even nail its leadership, but the SEALS cannot hold a city or a countryside by themselves. You use the 1st Infantry division for that.
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The proportion of leveled characters and higher CR NPCs in a D&D world is DM/world dependent.
If the party is one of 3 to 5 similar adventuring parties throughout the entire world then it might not be strongly considered as a possible issue when deploying troops. Leveled characters would be just so uncommon it isn't worth considering.
However, out of a population of 1,000,000 ... if just 10% of the population (1/10) has some capabilities it gives at least 100,000 individuals with class levels or equivalent. This could means a minimum 10,000 individuals into tier 2 and tier 3 (if only 1% of the population achieves higher levels or equivalent). If an army is 35,000 ... you could potentially have it composed of entirely leveled characters or CR1/2+ creatures since the army may represent better prepared fighters with training (leaving aside untrained conscripts or levies). Note: for comparison, the population of medieval Europe was about 50 million and the Roman Empire was 50 to 70 million people.
So, when I DM a D&D world, small groups of comandoes composed of elite individuals would certainly be a consideration when deploying troops. Other DMs would have other opinions and their worlds may differ.
However, just looking at the monster manual section on NPCs (and of course a DM can use whatever they want).
Berzerkers CR2 - "Hailing from uncivilized lands, unpredictable berserkers come together in war parties and seek conflict wherever they can find it."
One might reasonably expect entire warbands of Berzerkers to be a possible threat.
On the military side you have:
Guards (CR 1/8) "Guards include members of a city watch, sentries in a citadel or fortified town, and the bodyguards of merchants and nobles."
Knights (CR 3) "Knights are warriors who pledge service to rulers, religious orders, and noble causes. A knight's alignment determines the extent to which a pledge is honored. Whether undertaking a quest or patrolling a realm: a knight often travels with an entourage that includes squires and hirelings who are commoners"
Scout (CR 1/2) "Scouts a re skilled hunters and trackers who offer their services for a fee. Most hunt wild game, but a few work as bounty hunters, serve as guides, or provide military reconnaissance."
Veteran (CR 3) "Veterans are professional fighters that take up arms for pay or to protect something they believe in or value. Their ranks include soldiers retired from long service and warriors who never served anyone but themselves."
The MM could really use a CR1 or CR1/2 military type NPC like a solider. Untrained conscripts could be represented by commoners. In any case, veterans represent experience soldiers and would likely be NCOs and officers. More challenging military ranks could be filled using the Gladiator NPC. Knights would also have officer roles. You could have exceptional units composed mostly of veterans but those might be uncommon or rare. Depending on the context, these NPCs could be a reasonable choice for putting together a military unit. Each squad of 10 could have 7 guards, a scout and two veterans for an experienced and trained unit. Organize the army from squads to companies to brigades ... add in support in the form of mages, priests and acolytes with the occasional druid or two tacked onto the recon and support company or squads. With an army organization in mind it becomes easier to figure out what forces might be assigned to what tasks ... then just let the party face whatever is sent. You don't want the players to get the idea that the world revolves around them :)
I was specifically referring to the dense troop formations you see in a lot of cinematic battles, and which were historically used at times (e.g. the Greek phalanx, Roman infantry tactics, pike and halberd formations). The battle of Helm's Deep in the movies had the orcs using dense multi-line spear formations (basically equivalent to pike formations), and those are just meat for attack spells.
*Warning this is extremely euro-centric because that's the history I know*
There isn't a sensible distinction to be made between Classical/Medieval massed formations and Napoleonic War/American Civil War (and to an extent WW1) massed formations in the context of this discussion. An infantry square is a tercio with no pikes, a tercio is a schiltron with added arquebus/muskets, a schiltron is a phalanx that happens to be in the medieval period. The specific weapons being used differ, but we have now almost 2,500 years of recorded history in which people have fought using an evolution of essentially the same tactics.
All these formations were used on battlefields that had weapons that were capable of dealing with massed troop formations, so why did massed troop formations stick around for so long?
Because you were still a lot safer, and more effective as part of a massed formation than as an individual. Change out a battery of field guns for a squad of wizards if you like, but I don't see any fundamental difference in how a cohort/company/insert other organisational term here of infantry are going to deal with that - they are either going nowhere near them and calling in skirmishers or cavalry or charging them down to negate their advantage.
As to your example of the mage using magic shenanigans to run away - at that point he has lost. He might have killed 30-40 people, but ultimately he has retreated because the enemy have attacked him with a force he cannot withstand.
The sensible distinction is that they were largely ineffective and a tactical mistake as of the late 19th century. Now, people do sometimes persist with obsolete ideas for a while in the face of new developments, but all evidence is that broad area spells are not new developments in typical D&D settings.
First of all, the fact that the mage has retreated does not mean his supporting troops have retreated. Secondly, it's only a loss to retreat if there's some particular value to holding your current position, but there are plenty of skirmishes where your primary objective is destroying enemy forces. If your goal is holding ground you probably use a different spell mix.
A mage facing a large force and only able to destroy 20 at a time is going to be at it for a lot of days, or even weeks, to have any real effect on a big army. By then the enemy force might have already captured its objective or overrun it.
As a general rule, killing lots of enemies is inferior to taking objectives, which is why Patton did so well in WW II vs the less effective generals like Montgomery, who were trying to "wear down" the Germans. Yes, Patton's army did achieve a higher kill rate, but he also out-flanked, surrounded, and out-raced the German army to multiple objectives by using his army's mobility (which is why he achieved the superior kill rate). I don't think Patton would have cared a whit if some grenade-chucking enemy (i.e., the mage) were plinking at his army while he was capturing a city. This is exemplified by my uncle, who fought in Patton's army and quoted him as having said, "I don't care how many dog tags you have to bring back, I want that hill." A direct quote from the person who is considered one of the most effective, if not the most effective, generals in the history of warfare.
The mage who is using hit and run tactics could have made Patton's army bleed a little, but hit and run never took a hill (and more importantly, held it).
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Really? Things were certainly beginning to swing away from massed formations by the late 19th century but you cannot seriously make an argument that infantry armed with, say, chassepot breech-loaders and supported by mitrailleuse are capable of the sort of squad-based maneuver warfare that would become the standard by the Second World War - one troop of cavalry comes their way and they are all toast. Conflicts in this period were exceptionally bloody, but we need to remember the context that these were literally the largest armies that history had ever seen - none of this should take away from the absolute horror and pointlessness of these conflicts, but how else are you planning to fight these wars before motorisation and the light machine gun. The Prussians during this period certainly began to innovate maneuver warfare, but the main reason it was successful was the sheer number of people they could put into the field.
I was assuming that these hypothetical troops were commanded by someone competent and wouldn't decide to charge randomly at anyone they happened to see. If there is no tactical advantage to the position the mage is holding, why would they even bother to approach? Why is the mage even there? Also, the main objective of skirmishes is almost never simply to kill people - the main objective is usually securing or screening the deployment of the main force. Like BioWizard said above, a good commander is concerned with objectives - both strategic and tactical - and assuming the mage isn't a complete moron and standing in the middle of a random unrelated field, we just made them move off the objective they were holding.
Mages are something akin to artillery. Warfare has had cannons, catapults, and other forms of artillery for a long time. Or perhaps not even that ...
A modern day hand grenade has a thrown range on the order of 100' and a lethal radius on the order of 16'. This is remarkably similar to a fireball with a range of 150' and 20' radius.
https://en.wikipedia.org/wiki/M67_grenade
Hand grenades are ubiquitous in modern warfare. Many troops might have one. A level 5 mage is almost the equivalent of a soldier with 2 hand grenades to throw. Is their impact on a force really that devastating from an actual damage point of view?
Anyway, in order to get within fireball range, that mage is within 150' of their target. Longbows have a normal range of 150' and long range of 600'. If a mage reveals themselves by casting a fireball, then they will become an immediate target for every archer within range. Having a strong long bow force then becomes a clear counter measure to enemy mages though having some of your own mages would also be a good idea.
It can be interesting discussing the effects of magic on how warfare would develop in a D&D world ... but most of the 5e D&D combat rules are aimed at small scale skirmishes and not at larger scale warfare. However, one of the big changes between 5e and earlier editions is that previously a high level character might be able to take on an entire army and expect to win while in 5e a level 20 character might be defeated or at least forced to flee by 100 low level NPCs.