December Content Update: The Measure of an Empire (Patreon)
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How big should your fictional empire be?
That's a question which is actually two questions: a worldbuilding question, and a storytelling question, with the answer to the latter building upon the answer to the former.
So let's start with the worldbuilding half, which is the question of how big an area any government can be in your setting, on a practical level.
Ultimately, any sort of government (be it totalitarian, democratic, of feudal) relies to varying extents on three networks of control: cultural, bureaucratic, and military. The last of those is the most self-explanatory. The oldest and most basic responsibility of what political theorists call "the state" is the maintenance of internal and external order through the monopoly on the use of force: which is to say, it must have the biggest stick to beat down not only potential rebels and political opponents, but external threats like foreign invasion as well. It seems almost a given that any kind of polity would need a way of maintaining and deploying armed forces to trouble spots in both sufficient numbers and quality, but with sufficient speed as well. An empire which can't defend its outer territories from outside threats will lose those territories, either to foreign conquerors, or locals who've decided that there's no point in continuing to owe loyalty to a metropole which doesn't protect them.
Of course, just because military control is the most obvious network of state control doesn't mean it's the only one, or even the most important one. Those of us who live in vaguely functional states which have been at peace with our neighbours for a long time don't really see evidence of military control at all. Indeed, we tend to see the idea of troops on the streets not as a sign of strength, but of weakness - of a government which is so afraid of its own people that it must rely on guns and jackboots to keep itself in power - the mark of a regime which most of us would consider authoritarian, if not totalitarian; and therefore dysfunctional, malevolent, and unstable.
For a lot of us, that means we tend to see the other two networks of control more often: the bureaucratic and the cultural.
States are not just armies. They are also codes of law, systems of taxation, public services, and communication. These elements comprise the state's Bureaucratic network of control. Through the assignment and deployment of credentialled representatives - bureaucrats - the state can institute what we often refer to as "public policy", the parts of governance which interface directly with the governed. Ideally, these policies benefit both the state and those it governs over: taxes fund public services and common defence, uniform codes of law allow for the judgement of crimes and disputes without starting private feuds, systems of communication allow for different parts of the country to redistribute resources to each other - for example, allowing taxes levied from one province to fund an army defending another.
But for Bureaucratic control to work, it needs to be something which the people paying the taxes and filling up the ranks of the armies need to see as a net benefit. This is where Cultural control comes in. This doesn't necessarily mean that each constituent region needs to have the same religion or national identity as the metropole, it simply means that at the end of the day, the people on the street believe that they're better off with the state in charge than without it - that they believe the public services they get are worth the taxes they pay, and that the armies they fund and people aren't pointed at them more than towards a common enemy.
All three of these networks of control are essentially subject to logistical bottlenecks, ones governed by technology and infrastructure. Cultural control relies on the flow of information from the metropole to the periphery, to ensure that everyone's on the same page - at least about the government and its policies. This means that its limits are the speed of the ability to send words and pictures. An empire which has telegraphs or sending spells can spread that network far faster than one reliant on horse couriers. Likewise, bureaucratic control relies on the ability to send those bureaucrats to where they need to go, and to both resource them and discipline them if any should step out of line. That relies on the ability to move small groups of people or small cargoes quickly: the quality of roads is a huge part of this, so's shipbuilding, and the availability of infrastructure like relay stations for horses.
Maintaining military control is perhaps the most difficult of all, because armies aren't just a lot of people and their equipment, they're also all the supplies you need to keep those people armed and fed. This means you need infrastructure capable of through-putting massive amounts of cargo: wide roads, large cargo ships, big magical portals, railroads, starships. Armies (and fleets) are an absolute pain to get from point A to point B. Moving large armies over land, especially before railroads, is a painstakingly slow process for a whole variety of reasons - doubly so if you don't want to lose three quarters of that army to desertion, disease, and starvation along the way. There's a reason why so many empires built immense infrastructural networks to allow themselves to redeploy and project their military control - be it Rome's military roads, Britain's coaling stations, or even the modern US military's wide network of overseas airbases.
This is all important because each of these three networks reinforce each other in a way which ensures that the failure of one will make it easier for the others to fail. Bureaucratic failure means a government can't collect taxes, impose policy, or maintain discipline within its own power structure - which means the governed lose faith in the ability of their government to function, the armies don't get paid, and regional administrators and military commanders begin thinking of themselves more as local warlords than public servants. Cultural failure means taxes don't get paid, armies don't get recruits, and the ones that do show up are less likely to believe in what they're fighting for. Military failure means that the people on the frontier don't feel like their taxes are protecting them, and that the central government doesn't have the force to keep its own local bureaucrats and other powerbrokers in line.
What makes all this more complicated is that the criteria for "failure" isn't set either. These networks of control aren't being graded on a scorecard, but they're being compared with potential alternatives, not just internal (in the sense of regional particularists who think a given area might be better off by themselves) but external ones (if people think another country can offer a better deal than their current one, they might as well switch sides en-masse). Losing these competitions is generally what brings empires down: outer provinces think they're better off independent, or the metropole no longer thinks they have enough in common with the periphery to bother protecting or providing services for them. These are the failures which often get described as "decadence" or "corruption" or "internal rot", and almost every empire in history has met its end at least partially because of them.
Note that this isn't to say that an empire which fails to maintain one of these networks is one which can't exist. Almost every state's networks of control exist in an active state of complex competition with its competitors, a competition which the state "wins" or "loses" on the case-to-case basis of an individual citizen. Some people will support a government no matter how badly it fails them. Others have standards so high that no state could ever really fulfil them. What a state tries to do is maintain those networks of control over as many people within the territory they want to claim as possible, so that those who support the government and accept its control will always outnumber those who prefer a different alternative.
If you look at history, you'll see that a lot of previously common systems of government actually kind of accept that they're going to fail at maintaining certain elements of control and are structured accordingly. Feudalism (or Manorialism) for example, simply assumes that a central government will never has Bureaucratic control, so instead it co-opts the local elites who will. In short, in a Feudal system, the central government's networks of control only extend to landowning aristocracy, who in turn impose their own networks of control over their own vassals in a way which means the central government has very little direct control over them. The result is a system which is in many ways sacrifices breadth for depth: while a central authority can maintain sovereignty over a much wider area than they would through direct rule, they also must deal with nobility who are constantly incentivised to go independent with the people and lands they have direct control over. It's telling that the medieval and early-modern Kings of France and Holy Roman Emperors spent more time at war with their own nobility than with outside threats.
This is all to say that the failure of a government to maintain one of these systems of control doesn't necessarily mean it's doomed. It just means that there are obvious sources of instability which lead to conflicts between central authority and local interests, which might lead to the disintegration of the realm, or a might not - which is to say it creates conflict and uncertainty. Those are terrible things for the fictional individuals who would live under that society, but for you the storyteller, those are going to be your bread and butter.
Which leads us to the second half of the question. We've covered the principles which determine how large a fictional state can be, but we haven't covered how big it should be relative to those limits which your setting's technology, infrastructure, and geopolitics allow - and how remaining within those limits, or exceeding them might serve the story you want to tell.
What does that mean? It means that if you want to tell a story set in a Empire which is already in a state of fatal decline, then you can make that Empire one which has exceeded its means to maintain its own stability. Make its frontiers too far from its capital. Make its armies too distant to respond to threats effectively. Make its bureaucracy unable to deliver the public goods and make its people identify more with their hometown or the local warlord than a distant central authority. Maybe you want to sent your story in an Empire which is still conquering, is losing its ability to keep expanding? Then likewise, give it a size it can't maintain, but make it so those in charge don't realise that they've exceeded their grasp until you need them to.
This goes the other way too. Say you want to create a society which is well-run and efficient. Then you'd be best encouraged to keep it within the limits of what it can maintain. Perhaps those in charge are actually aware of what those limits are precisely - which is a great way of showing off that the political elites aren't willing to let their ambition get ahead of them. Maybe they would normally have exceeded the limits which are set by their technology, but they've developed some clever way of extending those limits for one network or another - even if that means overtaxing the others. For example, maybe the Empire maintains wide networks of military control by stationing armies in the provinces - but they're too distant to be kept paid and equipped by the central bureaucracy - which means they have to rely on, and begin to identify more with the locals they're supposed to be policing over the distant central authority they're supposed to answer to.
If you want conflict, then having one network of control weaker than the others provides a wealth of good story hooks. If military control is weak, then you have the perfect setup for a foreign invasion. Cultural control is weak? Peasant rebellion. Bureaucratic control is weak? Disloyal generals and local administrators, leading to civil war. Those are just a very narrow range of potential stories for very specifically stories that require some kind of armed conflict. There are easily just as many possibilities if you want to tell a story about class inequality, ethnic tension, religious strife, or financial corruption.
So to sum up: how big can your fictional empire be? As big as the available technology, infrastructure, and potential competition allow it to be. How big should it be? That depends on both the answer to that first question, and the nature of the setting you want to set your story in.