To worldbuild a fantasy military, start from the un-glamorous constraints rather than the cool units: who pays the soldiers, who feeds them, how far they can march before they starve, and what your magic system can and (more importantly) cannot do on a battlefield. Armies in fiction fail the same way they fail in history, through money and logistics rather than through a lost duel, so a believable fantasy military is one whose size, organization, and tactics all follow from the society that fields it. Then write the war through the people in it and record the campaign somewhere you can check it, because a long serial's war has to stay consistent across hundreds of chapters or readers stop believing in the world.
How do you worldbuild a fantasy military?
You worldbuild a fantasy military by treating the army as an instrument of the society that raises it, not as a box of cool units you drop onto a battlefield when the plot wants a war. The way I most often see this go wrong is that a writer designs the fun parts first, the elite legion with the black armor, the dragon riders, the mage corps that throws fire, and never asks the boring questions that actually decide wars, so the army has no visible means of support, no reason to be the size it is, and no logic to how it fights beyond what looks good in the scene. The fix is to build from the bottom, from the farms and the treasury and the roads, and let the glamour sit on top of a foundation that holds weight. An army is the most expensive thing a pre-industrial society can own, and everything about it, its size, its organization, its tactics, even its loyalty, follows from who can afford it and what they can feed it. This is the same pressure-point method I use for every hard worldbuilding problem in the guide to world-building for web fiction, and warfare is where skipping it shows up fastest, because readers who know a little history can feel a fake army the way they can feel a fake accent.
My feeling is that the military is also the place where a fantasy world's politics become physical, which is why it pays to build it in conversation with the powers that wield it rather than in a vacuum. The army is the sharpest tool a faction owns, so who commands it, who pays it, and whether it will actually obey an order are political questions long before they are tactical ones. Get that relationship right and a war stops being a set piece and becomes the continuation of the story's politics by other means, which is more or less what Carl von Clausewitz argued in On War (published 1832) and what every good war story quietly understands.
Start with who pays and who feeds the army, not with the cool units
A fantasy army is governed first by economics and logistics, because the limit on how many soldiers a world can field is not courage or steel but the surplus its farms and its treasury can spare. Historians have long pointed out that pre-modern armies were capped by what they could carry and forage, and that an army which outran its supply lines did not fight gloriously and lose, it simply dissolved as men starved, sickened, and slipped away home before a battle was ever joined. There is an old line soldiers like to repeat, that amateurs talk tactics while professionals study logistics, and I think it is the single most useful idea you can take into building a fantasy war. The Roman legion, a standing professional force of several thousand men, existed only because Rome had the roads, the granaries, and the tax base to keep it fed and paid year round, where a feudal lord relying on a seasonal levy of farmer-soldiers had to send them home for the harvest or watch his kingdom starve the following winter. That difference, standing army versus levied one, is not trivia; it decides whether your realm can wage a long campaign or only a short raiding season, and it should fall directly out of the economy you already built, the one I walk through in how to worldbuild a fantasy economy.
So before I design a single uniform, I ask where the money comes from, and the answer reshapes everything downstream. A merchant republic can hire mercenary companies and will, which is why Glen Cook's The Black Company (1984) reads like a story about a small business that happens to kill people, all contracts and payroll and unpaid invoices. A sprawling empire with good roads can keep legions in the field for years. A patchwork of feuding lords fields armies that evaporate when the planting season comes. George R. R. Martin builds much of A Game of Thrones (1996) on exactly this, where the question of who can feed an army through winter, and who is quietly going broke paying for the last war, matters as much as any sword. Decide your army's budget and its food supply and you have decided, without naming them yet, its size, its reach, and its breaking point.
Decide what your magic cannot do in a war
The defining choice in any fantasy military is not what your magic can do but what it cannot, because an unlimited magic system erases every constraint that makes war interesting and collapses the whole thing into a duel between two wizards off-screen. If a single archmage can flatten a city wall, a scryer can watch the enemy camp like a map, and a healer can put the dying back on their feet by morning, then forts are pointless, scouts are obsolete, and casualties stop meaning anything, so the careful logistics you just built no longer decide a thing. This is why I think the most convincing fantasy wars run on magic that is rare, exhausting, costly, or narrow enough that ordinary soldiers, walls, and supply trains still carry most of the weight. Brandon Sanderson's work is the usual teaching example here, and on the world of The Way of Kings (2010) the truly battle-breaking powers are scarce and expensive enough that armies of ordinary spearmen still have to march, hold a line, and die in the mud, which keeps the war legible even with magic in it.
My advice, then, is to write down the rules of magic in war before you write the first battle, the same discipline I argue for in how to build a magic system, and to ask of every power the blunt question, why hasn't this already ended every war in the setting's history? If you cannot answer it, the power is too strong and your military will read as theater. The flintlock-and-sorcery novels are a useful model for integrating magic without breaking it: Brian McClellan's Promise of Blood (2013) and Django Wexler's The Thousand Names (2013) both build armies where muskets and drill and supply still matter, and magic is a specialized arm rather than an off switch for the whole war. The trick is always the limit. A magic system with no cost is a plot solvent, and nowhere does it dissolve a story faster than on a battlefield.
Organize the army the way its society actually works
How an army is structured, who ranks whom, who gives orders, and whether those orders are obeyed, should mirror the society that raised it rather than a generic modern chain of command copied in without thought. A professional standing army develops real ranks, drill, and a culture of discipline because it exists full-time and can afford to train; a feudal host is a fragile alliance of lords who each brought their own men and each expect a say, which is why medieval battles so often turned on a noble deciding, mid-fight, that this quarrel was not worth his life. That distinction is where the military meets governance, and it is worth building alongside the realm itself, because raising and keeping an army is one of the central problems of running a kingdom that I take up in kingdom-building in web fiction. An emperor with a paid professional army can command obedience a coalition of jealous dukes never can, and that single structural fact can drive a whole plot.
The organization is also where you decide how large the world even thinks in. The Mongols are the instructive real case, an army organized in clean decimal units and built around riders with several remounts each, which gave it a strategic reach that terrified every settled empire it met; the structure was the weapon. A fantasy equivalent might organize around whatever your world uniquely has, mounts that fly, soldiers who can march through a portal, a mage attached to each unit for signaling. The point is that the shape of the army tells the reader what the society values and fears. 'We fight in the old way, every man under his own lord' says one kind of world, and 'the Legion does not ask your name, only whether you can hold the line' says a very different one, and both of those sentences are worldbuilding doing its job in a single line of dialogue.
Write the war through people, then keep it honest across the serial
A war becomes real to a reader not through the general's map but through the narrow, frightened, confused point of view of someone inside it, which is the difference between worldbuilding the military and writing the battle. The worldbuilding decides what the war is, how big, how supplied, how magical; the scene decides how it lands, and a good battle scene shows a viewpoint character only the few yards they can actually see, the ringing ears, the broken order, the rumor that the left flank has already run. Steven Erikson's Gardens of the Moon (1999) and the Malazan books that follow are built almost entirely on this soldier's-eye view, all marines and sappers and the grinding boredom between horrors, and Joe Abercrombie's The Heroes (2011) stretches a single battle across three days seen through the people trapped in it. That zoomed-in craft is its own skill, and I treat it separately in how to write LitRPG combat scenes; here the job is only to make sure the system under the scene holds together.
And in web fiction, holding together is the real test, because a war in a serial is not a set piece you write once but a campaign that may unfold over months of real time and hundreds of chapters, and the reader keeps a ledger. Running a web fiction platform, the authors I see manage a long war well are not the ones with the most elaborate order of battle, they are the ones who wrote down the load-bearing facts and could check them mid-chapter: which lord brought how many men, who died at which siege, how long the march to the capital actually takes given the supply you established, and where each commander stood when the alliance fractured. That is the same continuity discipline I argue for in keeping continuity in a long web serial, and a war multiplies every way it can go wrong. On IlorisNovel that record lives in the connected wiki beside the editor, where each army, commander, and campaign is a lore entry you can @mention while you write and lay out on the timeline as the war's events accumulate, so the siege you described in chapter sixty still has the same garrison, the same weather, and the same survivors when it echoes back in chapter two hundred. If you want to feel how that fits together, you can try the editor with no account and sketch a campaign into it. Pirateaba's The Wandering Inn, an ongoing web serial, is the web-fiction-native proof that this scale is possible, sustaining large wars and sieges across an enormous length precisely because the world's facts stay consistent as the body count climbs.
A fantasy military, in the end, is not a parade of impressive units you line up for a climactic battle; it is a consequence, the physical shape of everything your world already is, its farms, its treasury, its roads, its magic, and its politics, all expressed in iron and bread and fear. Build it from the constraints up, decide early what your magic is not allowed to solve, let the army's shape mirror the society that bore it, and then write the war through the people small enough to be afraid of it, and your battles will carry a weight that no amount of clever tactics bolted on afterward could ever give them.
by Jacob Tam · October 15, 2026
I run IlorisNovel, a platform for web fiction writers. If you want to see what building a world here feels like, you can try the editor with no account.