No More "Peasant Armies"!!!
Here's a notion I wish would die already: Medieval armies were made up of dirty and scared peasants armed with shitty spears, ready to be mowed down by the nearest knight for sport because we're all idiots here and military leaders have both no concept of strategy, and a callous disrespect for life.
Well, the last part isn't super off the mark, but think about that for a little bit more. You're a medieval king, your kingdom does not have that many people simply because of medieval demographics. Do you send to war:
John, 32, serf, has never picked up a spear in his life and has absolutely no motivation to simply not break and run into the forest as soon as he hears the thundering of hooves getting closer, OR
Sir Jonathan, 22, does nothing all day except ride a horse, pog, and train; is a fourth son of the nobility, has no employable skills, and knows that if he doesn't hit it big enough to be noticed by a senpai, he WILL lead a life of poverty and his children will fall from the rungs of nobility FOR ALL ETERNITY?
The choice makes itself. Throughout most of the later medieval period, fighters weren't whatever random asshole you could pluck from the field because YOU NEED THOSE FUCKERS TENDING THE FIELDS!1
The medievals simply knew that bringing untrained men into war was stupid as fuck - not because they were kind, but because they wanted to win the damn war as fast as possible so they can get delicious loot.
I see some of you in the back with the smug faces going "hehe but what about the infantry? Have you thought of that? I know that during Edward I's invasion of Wales he had around 15k dudes only from infantry, Crusader Kings told me."
First of all, good on you for reading. Second of all, the peasantry is a whole lot more complicated than that. People have this bad habit of thinking that you were either a shit farmer wallowing in dung who never picked up a spear before (like John above), or living in the lap of luxury with your hot wife and her many boyfriends. Does that sound like SOCIETY to you?
Medieval society had a spectrum of Dudes who went from "Cottar serfs who don't own their own turds" to "The richest man you can think of right now". Some of those dudes were "Peasants who can afford weapons, know there's money to be made in war, and have the luxury to go out and sign themselves up for Edward I's next big break because they can invest this money into better equipment and maybe push themselves into esquiredom."
And once you're there, you hear there's a different war your buddies are going towards, and another, and that's how you get mercenaries!
"But why did medieval texts call them, like, 'a company of ornery Welshmen / Flemish who suck and should die'?"
Because those texts are usually written by monks and other churchmen who don't have the time to stop the army and ask if each of their members is in fact Welsh, so he just assumes they're all filthy fucking foreigners who came here to steal our shit and would be better off as far away as possible.
Remember: you need WEAPONS to fight. No one will buy weapons for you; what, you think this is a charity? Why would I, the king, commission my blacksmiths to make swords for you, a person who will run or die as soon as the enemy show up, when I have both rich dudes who own horses and know how to fight AND peasant dudes who own bows and know how to shoot? It's plain bad business.
How do I apply this to my game, as a player?
If you're playing a fighter, here's a few considerations:2
What was your position in life that allowed you enough leisure to learn how to fight? Gentry have leisure built in, but peasants sometimes hail from military families or yeomen or whatever, people who can take a while off work to shoot a bow or beat a straw dummy with a sword.
Where did you get your weapons? "I found it on a corpse" is acceptable but dead men lying on the ground distributing swords is no basis for a warrior. This stuff is kinda expensive but not enough that you wouldn't be able to get it.
You signed up to work with someone. Either this happened of your own free will or you were born in a class of people who just fight for a living. In England, this usually happened by you getting wind that the army is mustering (this could be announced up to 6 or 7 months before the campaign start) and basically been told "Be there, payment is regular, you get to keep whatever you get your hands on during the battle, good luck."
You did not, or could not, return to civilian life. Either this was good money and you decided you could hack it for a few more battles, or you noticed you had some presence and thinks you could lead your own men some day, or there's just nothing for you back home except hunger and an uneventful life.
Always ask your DM who are these men fighting for and what is their ultimate goal. Warriors in the medieval period didn't hide their shit, they WANT you to know why they're beating you up, because if you can give it to them without a fight, so much so the better.
How to apply this to my game as a DM?
A few things to consider from behind the screen:
Consider the uncomfortable position that rich peasants and poor nobles both are in. What happens to third and fourth sons of the nobility? What happens to a peasant who's a good shot and who shows up to the muster with a longbow? That's a valuable fighting man who can't just be turned away.
Weapons come from somewhere. Armourers aren't merchants, they have a really useful skill that every single noble has demand for, they don't have to sell shit to anyone they don't feel like.
What's the current war? Medieval polities were constantly feuding over stuff. If they stop, the size of the houses shrink as there's less need of fighting men to keep around, and those guys are going out into the countryside to get their loot where they can. I.e. bandits.
What can a warrior who isn't at war do to keep their skills sharp? In real life, tournaments were very practical training, and they were also a good way to make money as you would have to surrender your armour and horse if you're caught (or pay their equivalent value).
The king usually asks for a certain numbers of soldiers of his nobles and it's up to them to figure out how they're going to find these people. That's how you get so many mercenaries.3
Wealth is to be spent. Historically, kings don't like sitting on top of piles of gold unless they're making a war chest. They WANT to spend that money on followers as gifts and shit like that because the most important currency of the middle ages is LOYALTY. If you can make someone rely on you for their income, you're creating a material interest in them having you around and in power. King John is remembered as a shit king in part because he demanded too much money from the nobility and didn't give back enough. This is a reciprocal system, the middle ages were NOT the Enlightenment, when kings were absolute and their word was law. Their word is very much not law, their land is.
Don't be afraid of having your encounters be "someone". Your bandits don't have to be a bunch of faceless, armourless, gormless goons ready to be killed, they can in fact be knights down on their luck, knights who aren't down on their luck but are content in "demanding extra tolls" from foreigners passing through their land, off-duty mercenaries who got too used to just grabbing the stuff they want without asking for permission, etc. Call them the Band of the Boar and voila, your bandits now have a personality and are a faction.
Consider urban militias. These aren't goons either, they're citizens (a very loaded term, usually "a guy who's lived in the city for more than a year and also has gainful employment") whose duty includes guarding the gates, patrolling the roads and bridges that are in the purview of the town, muster a posse to assist whoever is the local bigwig to hunt down criminals, maintaining the armoury, patrolling at night, firefighting, etc. They MUST own their equipment and sometimes they're semi-professionals being locally hired to take the burden off of the productive citizenry. These are great sources of manpower for a king, but justifying why you need the London Crossbowmen to cross the channel and attack Savoy is a big ask. Still, if it's a defensive war, remember these guys. Despite what Discworld would have you believe, the night watch is in fact made up of proper law-abiding citizens.4
Edge Case: Early Medieval Societies
There's always exceptions and weird edge cases. Maybe your medieval game looks less like 1300's England but rather with 700's England, in which case surely you'd have a bunch of peasant goons with spears right?
NO!
If your society resembles the Anglo-Saxons or any number of pre-year 1000 Germanic societies (or even Celtic as the Irish aren't that far from this), they likely don't have such a rigid need for "people having to be born with the right blood to be aristocrats". Instead, they likely follow a more pragmatic "as long as you have the equipment, you can be a warrior" ethos.
You can see this clash of "who is in the army" in the Battle of Hastings:
On the Saxon side, following the Godwinsons, you usually had an army following their local magnate - either an earl, bishop, or sheriff - or personal followers of these people. The first kind were usually thegns, landowners of varying wealth; the second kind were huscarls, basically bodyguards who lived on their lord's land as professional warriors and errand boys.
On the Norman side, following William the Conqueror, you had between 1000 to 3000 mounted men and the rest were mostly infantry. These were either mercenaries "from" Brittany and Flanders, or armoured foot soldiers. Remember, if they had armour, they had gotten it from their own pocket.
In an Early Middle Ages / Late Iron Age style setting, your army is most likely seasonal warriors and retinue-followers like the Saxons had.
Conclusion and Idle Thinking
Either way, stop putting those peasants who don't know what they're doing in the frontlines. It makes your kings look stupid and incompetent, your nobles don't have anything to do so they look like assholes for the wrong reasons, your peasants look like assholes because why are they not questioning this and turning the spears around on their lords, and you turn every war into a total war for the spirit of the country. This is NOT World War 1.
I suspect part of the reason this happens is because people want to portray their nobles as assholes and their peasants as victims, so they pick "mass conscription" because it would be scary for them in the modern world and run with that.
In fact, peasants did suffer on the hand of the nobility for other reasons. They paid taxes for one, something the nobility didn't do, and a common way of demonstrating that you won a given war was by pillaging the land.5 This has the added benefit of both making your dick look bigger because you won the war, but also making your opponent lose face with their followers: after all, why is he holed up in a castle while some dickweed prances around trash talking him to the four winds? Whatever happened to the oath he made to PROTECT us?
If you assume that the peasantry were just a bunch of victims held captive by an armed elite, you miss out on the very textured and complex reality that the peasantry could and did revolt, and could and did collaborate with invading armies if their current ruler was being a tool.
Personally, I would rather portray a world like that over one made up entirely of passive people who just have violence done to them.
Further Reading
Coins and Scrolls, generally. Particularly the post about war and the one about the three estates.
Thomas Asbridge's The Greatest Knight: The Remarkable Life of William Marshal, the Power Behind Five English Thrones, for a VERY specific look into the life trajectory of a man who lived in the very height of Chivalry, started as a third son of the nobility and wormed his way into earldom.
For an even lower dude you can look into John Hawkwood, who started off as one of those land-owning peasants I mentioned, didn't inherit it, and had to make do with £25, some grain, and a penchant for violence. Unfortunately I haven't read a book on him yet, but I'm given to understand that William Caferro is one of the main authorities on the Italian Renaissance, and he has a book named John Hawkwood: An English Mercenary in Fourteenth Century Italy that historians seem to like.
Jonathan Sumption's Hundred Years War series, particularly volume III which has a chapter about "Men at Arms", and Chris Given-Wilson's The English Nobility in the Late Middle Ages. Both of them give great insight into how war actually worked in England and France in the 1300's.
Matthew Hefferan's The Household Knights of Edward III and S.D. Church's The Household Knights of King John both deal with war in the 1200's and particularly the role of the closest knights to the king, his household men. The first deals with Eddie 3, who was beloved, and the second one deals with Johnny, who was hated so much that dudes were ready to turn coat for France.
If you have more early medieval / "tribal" societies, you might want to look at Sean Davies' War and Society in Medieval Wales, 633-1283, as well as Seán Duffy's Ireland in the Middle Ages or T.M. Charles-Edwards' Early Christian Ireland. A LOT of RPG shit is half-stolen from Wales and Ireland and then painted with an ugly coat of paint to make them "pagan" and "exciting" instead of just accepting that these societies were just as Christian as any other, AND that they were very interesting by themselves.
Exceptions do exist, but you would usually levy the peasantry only during a defensive war when they know they have something to lose, or if you think it's gonna be a short war and some of these richer peasants are great archers and have their own equipment, so you can just pay them.↩
Needless to say, both of these sections only apply if you're playing or running a game set in a pseudo-medieval world.↩
Practical example: in 1173, Young King Henry, brother of Richie Lionheart and Johnny Lackland, rebelled against his father, Old King Henry. Did Daddy Henry call up on his nobles to beat the fuck out of his son? No, he hired 20 thousand Brabançon mercs and sicced them on the boy. Young King Henry's own 'army' was mainly a retinue of other men he had amassed by just going to tournaments and spending ALL of his money on his followers.↩
They do usually hate their job though, because it usually needs to be done by volunteering, and a few weeks fighting fires and patrolling the streets are a few weeks where you're not MAKING MONEY. This is why professionalising some guys is usually preferred, and cities tend to have the money to do it too.↩
Contrary to popular belief, kings didn't like relying on stolen food to feed their armies. It's too inefficient and risky. Instead they would buy grain on friendly territory prior to the campaign and put it on a supply train to march along with the army, supplementing it with captured harvests every once in a while, but ideally you wouldn't want to rely on it.↩