It isn’t enough to just get warm bodies and throw them onto a battlefield; that’s a great way to waste lives and lose a battle. But how do you turn a person into a soldier?
The process depends heavily on how warfare fits into the society. If you’re looking not at a soldier but at a warrior — which, as I said before, is someone for whom martial skill is a core part of their social identity, rather than a profession they engage in only for a bounded span of time — then it can start quite early. For medieval European aristocrats, Japanese samurai, the kshatriya caste in India, and other such groups, adult male identity was often bound up with being a warrior, whether they were at war or not. Because of this, learning how to fight was a key part (sometimes practically the only part) of boyhood education. They went onto battlefields at what we would consider to be shockingly young ages; a book I recently read on Sengoku-era Japanese history mentioned samurai participating in their first battles as early as twelve years old.
This approach isn’t wholly the province of aristocrats, though. In any place and time that’s particularly unsettled, lots of people (especially lots of male people) are going to know something about how to fight. As this piece by Lyman Stone argues, “frontier, exposed, tribal, frequently-invaded, and less settled people make better medieval soldiers.” (Note that “tribal” there isn’t some kind of pejorative: a society characterized by that type of structure is also highly likely to have frequent raiding and skirmishes between tribes, that a more centralized state would quickly move to suppress.) If you live on a farm in the peaceful heartland of your country, you’ve got no particular reason to know how to use a weapon . . . but if you might need to defend your home and your family on a regular basis, then yeah, it makes sense to put some time and effort into that.
Of course, that still doesn’t mean you’re going to be skilled. The farmer who fights off the occasional raider — or who himself is a raider in his spare time — isn’t on a par with the nobleman trained by an armsmaster from the age of seven, unless maybe that nobleman has been very lazy and failed to keep up his practice. And even then, the latter has the edge in quality of equipment. The frontier farmer isn’t starting from a dead halt, is all, which puts him one up on the farmer who’s lived a relatively combat-free life.
But this also depends on what country those two farmers live in. After all, armed peasants who know what they’re doing can turn around to become a threat to their rulers, instead of a defense against threats from without. Because of this, many states have done their best to prohibit even the ownership of weapons by the common folk, much less any training in their use. After the century-plus of constant warfare that was the Sengoku era, both Oda Nobunaga and his successor Toyotomi Hideyoshi instituted “sword hunts” with the goal of confiscating the weapons that had become ubiquitous during that time, so as to prevent dangerous rebellions and solidify the class distinctions between samurai and their lessers. It’s a common, and maybe even a necessary, part of state development: the state reserves to itself the right to wage war, and so only authorized personnel are permitted the tools and training for it.
Of course, doing that means you may find yourself lacking in skilled combatants when the need arises. That’s why the Assize of Arms of 1181 in England mandated weapon ownership, specifying the required equipment by social class — and simultaneously forbidding it to Jews — and a royal declaration in 1363 ordered every able-bodied Englishman to practice archery in his spare time. (As the old joke goes, to create an English longbowman, start with his grandfather.) So really, this can go either way. A lot depends on whether the ruler is more worried about threats from within or from without, and also on whether they’re able to maintain a standing army or not. The latter means you can rely on having a body of experienced professionals, instead of random schmoes who may or may not have spent their holidays drinking instead of shooting arrows.
Then again, what do you need your soldiers (or warriors) to do? The behaviors expected of someone in combat vary widely depending on the nature of the combat in question. If you’re raiding across the Scottish-English border for sheep, then the best approach is to come in quiet and get out fast, minimizing the risk to yourself. If you’re raiding an enemy tribe to demonstrate your prowess and improve your social standing in your own tribe, you might want to make a flashy display of charging straight at and defeating your opponent. If you’re a musketman in the ranks of an eighteenth-century European army, you need to be able to load, prime, and fire your weapon in coordination with the rest of your unit, and stand your ground under the enemy’s return fire. If you’re a modern soldier fighting irregular forces, you need to be able to operate in relatively small groups with a high degree of independence and mastery of quite complex technology.
These differences in how combat operates affect the creation of the people who carry them out. A random guy can quickly be taught how to use a musket, and in the massed-fire approach of that type of warfare, his marksmanship doesn’t matter too much; it’s more important that you train him to carry out those rote actions no matter what else is going on. Good luck creating anything like an effective sword-wielding cavalryman in the same span of time. The more complex the skills involved in combat, the more time needs to be spent training people for them — which in turn is only possible if that’s part of their social identity, or if you have a standing army. None of this, from the types of military forces to how they’re raised to how they’re trained, is separable from the other pieces around it.