Let's focus specifically on the devolution of empires. How is it that something so powerful falls?
The collapse of Rome, Byzantium, the Han, the Mayans and so on. What similarities can be seen in the fall of these great empires? Whether natural events, social trends, or technological advancements. Also, how can one use this information in building worlds?
There's a fascinating book seeking to answer this exact question called Day of Empire by Amy Chua. The gist of her answer revolves around tolerance, empires rise when they demonstrate it and fall when they don't.
Rome is the poster child of this hypothesis. Beginning, as she claims, as a bit of an underdog rogue state where outcasts of other places ended up, Rome had a diverse range of knowledge from all over the ancient world. And where most societies would marginalize these outsiders, early Roman society embraced them. Immigrants were a vital source of ingenuity and labor, this let them steal more than a few useful technologies from neighbors and consolidate themselves as a technological powerhouse. Most of her argument is sort of made in the context of modern America, so take it for what you will, but the early days of the Roman Empire were sort of bound up in that scrappy, underdog identification. But before they knew it, they controlled the known world and adopted the Persian governance model, until they realized that what defined a Roman citizen was vague. Hardships came as a solid identity was sought and subsequently enforced on everyone. Rebellions started to emerge and all the effort enforcing internally left them weak externally.
But she goes through a number of cases, exploring this trend. The Portuguese had a similar underdog status until they founded their navigation school and became a mecca of shipwrights and astronomers. This fueled a boom that led to an empire lasting almost to the present day in strategic pockets. The Dutch had a very similar start to their story, eventually inventing the modern economy before the grand daddy of empire England welcomed the Dutch themselves into their fold with the Glorious Revolution. England's story of tolerance includes the formation of the United Kingdom in the first place, and the historical status of Great Britain as a warzone for everyone from the Romans to the Vikings. There was a great deal of people calling the island home and national identity was very fluid. As they spread all over the world, they didn't try to usurp local power. In India, they ruled through the traditional ruling class. In Africa, they'd just pick a favorite chieftain and support him with guns and the like. In Egypt, they just added a military arm to an existing state.
The moral of her story was that you didn't need to be the biggest, best or smartest to rule the world. You just had to be willing to accept people who were. Most of the technological might of superpowers throughout history were not the result of nationals working in isolation, but collaborations often involving immigrants.
The second part of the argument was why would a historical pattern of tolerance leading to greatness ever devolve into a pattern of intolerance to tear the empire apart. In absolutely basic terms, a society needs some sort of glue to define it and make everyone feel like they're a part of it. Many empires have camaraderie aplenty to define them, oftentimes because they are an underdog in regional politics and historically beaten around. Rome was a gaggle of outcasts. Portugal had been battered around by Spain and the Moors for ages. The Netherlands had been the butt of Continental politics for awhile.
Great conquests that lack this kind of cohesion quickly fall apart, such as Alexander the Great. There was no effort to call his subjects Macedonians and when he died, his generals just carved up their own kingdoms. Rome offered a path to citizenship and made it look appealing. For awhile, there were a lot of traditionally Roman Romans to define the culture, but gradually, enough outsiders completed their service and were made citizens. Eventually, this led to the arguments we see even today about how adding such and such group detracts from the original character of a people. In a quest to define what made a Roman Roman, they ended up destroying themselves. Her conclusion was that this was the common tipping point for why a state would adopt a policy of intolerance and cut themselves off from the benefits of tolerant policies.
Societies advance when they become more internally cohesive, trusting, egalitarian and merit driven. They fall when they lose those traits.
An old blog post of mine expands upon this
Military success, increased trade, growth of science, technology and arts are all epiphenomena of the social and political changes.
Rome is a canonical example. Rome became a superpower because it was a representative republic with leadership and advancement based more on merit than lineage. Lineage based societies select leaders of all kinds based largely on lineage. You get to be king because your father was. If the leaders are idiots, your stuck with them until they die. Merit driven republics can choose leaders based on performance and remove them in the short term if the fail. Conversely, they can reinstated if proven correct e.g. Fabius vs Hannibal.
Rome also allowed a much higher percentage of it's population to bear arms meaning that it could field massive armies compared to surround populations. This gave Rome more military manpower per unit of population that cultures that allowed only a fraction of their population to be armed. Rome became much more militarily "dense" than its competing polities.
When the Republic fell, so to did merit promotion and high numbers of military service. The Empire continued by inertia but it was already doomed.
It's dispositive that that the Republic expanded massively compared to starting size, growing several times over from a mere county sized area to cover half the Mediterranean world in a little over two hundred years. The Empire expanded only a about a third and reached its maximal extent in the first century.
It's easy to see why. Whereas the Republic trusted it's soldiers as sought to see them well armed, trained and led, the Emperors feared them (rightly so) and kept them well paid, but poorly armed trained and led. The imperial armies grew more expensive and political while growing less and less effective. Worse, the percentage of the population who could be armed or serve plummeted. Rome relied more and more on hired auxiliaries who were usually "barbarians" with little loyalty to Rome.
From the time of Marius onward, the rule of law in Rome progressively failed with economic success relying more and more on success and connections in politics which in turn grew more and more violent. Romans spent progressively more time fighting each other than anyone else. Merit promotion died.
By the end of the second century Rome had inherited vast borders but had lost the social and political systems that fielded the armies to defend them. Trade faltered as property, contracts and profit became matters of political whim. With the fall of trade came less taxes and less military spending. Area after area become isolated and indifferent to affairs in the general empire.
In last century, "Roman" armies and their opponents where largely indistinguishable. Rome didn't fall to barbarians as much as become them. Rome had no technological, social, economic or political advantage over the surrounding societies and they just oozed over the borders.
In the end, Rome was just another rigid, hierarchal, lineage based society among many. It simply dissolved into the noise.
The pattern is repeated time and time again. Athens, Rome, Carthage, Genghis Khan, Venice, Florence, Lisbon, Swiss Cantons, Dutch Republic, England, America.
In each case, the societies became more merit driven with decentralized political power and surge in trade, science, art and military success. The success of some is just staggering. Venice and the Dutch Republic particularly stand out. Both were small societies built on river deltas who literally didn't even have land. Yet, they ended up dominating their regions and influencing much of the world.
But such societies do not seem stable. Their dynamic periods last only 100-200 years followed by decline into mediocrity. Meritocracy is counter to our genetic programming to favor our kin over others. In the end, riches, arrogance and complacency lead us to let our guard down and we revert to the lineage pattern.
I would note that societies generally never recover from decline. Instead the torch of progress passes to another. The Renaissance wasn't the comeback of Rome, it was isolated to a shockingly small number of people in a few Italian cities for under a century and then it promptly shifted to Northern Costal Europe. The progression went Venice --> Florence --> Lisbon/Geneva --> Dutch Republic --> England --> America --> ?
I think you should take a look at Toynbee's A Study of History, which tries to answer this question in detail. The theories may be a bit antiquated at this point since Toynbee was working prior to WWII, but in general he laid out a system in which civilizations are constantly faced with a variety of challenges, and their survival is dependent on the ability of a "creative minority" to effect some sort of change in response. A civilization fails when it is no longer agile enough to respond to that challenge.
The ages of history then can often be seen as a sort of call and response. If you take Japan vs. China, for example, the approach of the modern era forced both countries to respond in some way. Because Japan's power structure was more centralized and the entire country could be properly mobilized it was able to take advantage of modernization and became an Empire. Its pivot towards openness and expansion was the result of a creative minority that forcibly took power from the then-ruling bakufu. Had this particular creative minority failed to control the policy of Japan, the country may have ended up much more like China, where the bureaucratic system was unable to adjust quickly enough to the challenges of invading westerners and centuries of dynasty were ultimately ended.
In a way this theory is closely related to the concept of survival of the fittest. There are a lot of outside pressures on a civilization, but whether that civilization can survive will depend on its adaptability. But Toynbee goes further than that when he posits that the ability of the creative minority to avoid becoming a dominant minority is the primary factor in whether or not a civilization can survive through time.
At some point the creative minority will find itself with the largest portion of power in a civilization. It will naturally want to retain that power indefinitely, and will by begin to solidify elements of governance and policy that had before existed for practical reasons. The more effort dominant minority puts into retaining power, the less agile it becomes to deal with any other crises, and the more hostile the general population will become.
The pressure of this dominant minority will result in the creation of what Toynbee calls the internal and external proletariat. There will be those who abandon the society and become the "barbarians" of that civilization (easier to do back when you could literally walk away from a nation into the wilderness) and an internal proletariat that resists within the civilization. (examples of this being the Christians in Rome, the Nationalist and Communist forces in colonial China, the Protestant reformation, and so on.) This alienation is an important contribution to the failure of a civilization, because it means that the population from which a creative minority is gleaned is instead invested in resistance. In the case of Japan, there was still a strong allegiance to the notion of imperial authority, and the creative minority was absorbed into the existing structure, preserving the idea of Japan even as it pursued a radically different set of policies. China, however, was overcome by an internal proletariat that was opposed to the fundamental precepts of Chinese imperialism and the resulting military and cultural revolution resulted in a completely different nation. While some (particularly the Chinese themselves) might refer to the unbroken and ancient civilization of China, more cynical observers might say that the old Chinese civilization was cannibalized by an entirely new proletariat, and the current China is actually quite young. As with a lot of things in history, the exact line between one thing and another is sometimes fuzzy.
The tl;dr of this is, empires will undergo a number of stages in their development, and their decline is often marked by a crystallization of the dominant minority and increases in effort towards maintaining power and occupies itself with "worship of its former self," instead of focusing on creative solutions to external issues. This results in an increase of resentment and resistance by the disenfranchised population, and hinders its ability to respond to challenges that arise. Eventually one of those challenges will prove to be more than a civilization in this now weakened state can manage, and it will collapse.
As I said, this is an older, and not universally accepted, model, but I've found it a useful one to know. When you're building your own civilizations it's good to think of the following questions:
If you have these three things, you can see where points of particular conflict or stress might arise, and result in openings for a fatal blow to a civilization.
Many, many, many different things. But almost always something outside of that empire's control. A few examples:
This is a very common one - it can be large and powerful invaders like Alexander the Great and his multiple conquests across the world (See also: The Mongol Horde), or incursions from 'lesser' forces that use superior technology/tactics, such as the superior fighting skills of the Barbiarians (Gauls) that invaded Rome.
Equally possible is an uprising from within a society itself - slaves are often a cause of this (if you go with the bibical story of Egypt, Moses and his exodus certainly counts) but any downtrodden people within a society can cause similar problems (See: Christians for Rome).
Don't discount the possiblity of mother nature tearing the status quo apart. Disease completely upturned the nature of power in Western Europe (And severely softened Asia for that matter), and ran rampant across the Americas before Europeans came along. Any disaster, however, could be written in as a major population-destroyer. Floods, storms, floods AND storms, massive earthquakes, mother nature is a very fickle beast.
And let's not forget good ol' sloth. This is usually part of the disparity that leads to the downtrodden revolting against the upper-class, in partiuclar when the downtrodden include the members of the public that are supposed to lead the army. See Rome (Damnit Rome, you just can't hold onto your empire for more than 200 years, can you?).
Decentralization:
TLDR: Potential problems include (not in order): Bad capital position, capital too dependent on the regions, internal struggles at the court, bad administration.
My main example here is about the Tang dynasty. Decentralization or lack of centralization was a major problem. They were unable to keep the provinces in check (this weakened their power), but the fatal blow was a reduction of the food production.
Their capital was located in Chang’an (today’s Xi’an). It was a symbolic location since it has been the capital of other dynasties before them. It used to be the cradle of China but the economic center of the Empire had moved to the east. The city location was not ideal for a capital but the Tang preferred it because it was the base of their political support and for symbolic reasons. Having a good control on the capital region is very important and many rulers do not hesitate to change the location of their capital for a friendlier one when they founded a new dynasty. But the Tang kept this location (with some exceptions) at the expanse of losing the support of the regions. Most of the aristocrats lived only in the capital, not in the regions.
Note: the Tang knew about these problems. Some Emperors did move the capital to the east at Luoyang, but they always moved back to Chang’an after. That was very costly and foolish IMHO.
The capital was cut off from the border regions when the problems at the court became very serious. Example: one Emperor got murdered by the eunuchs. The faction struggles between the eunuchs, the bureaucrats and the Emperor lead to a couple of years without leadership from the central government. The job of ruling a province was initially a nomination form the capital for a fixed number of years. A weakened court made some local rulers able to make this position hereditary. They were also able to establish a firmer control on taxes and on the army. Rare are the regional leaders that were loyal to the Tang, most had their own goals. Most of them had literally no ties with the dynasty. As a result of these processes, the Tang were unable to collect taxes in most regions they presumably controlled. The regions had enough power to defy the Emperor and that they did during the An Lushan rebellion were the northern lords attacked the imperial forces. The Emperor managed to regain control but at a great cost and the sack of their own capital. The Tang had little authority over the regions and eventually, one general was able to replace the Emperor.
Note: The Tang are generally regarded and the greatest Chinese dynasty because of their cultural refinement not for their efficient administration. The later is more associated with the Song.
Note: Without good counsellors, a ruler is like a blind man and incapable of acting properly. He might not know what is happening in the regions and can be persuaded to choose certain options that are not in the Empire best interests. Bad persons can influence him and manipulate him, without but also with his consent when the ruler becomes a puppet. A strong Emperor with weak counsellors can also be problematic (Wilhelm 2 in Germany).
Lastly, another problem also weakened the Tang. The presence of a capital city with all the administration personnel and the army protecting the western frontier required more food that what this arid region could produce. They had to import it form the south but the low quantities of rainfall made the transport of goods on the water and rivers canals hazardous. This meant communication problems and a dependency on the other regions.
Gunpowder Empires:
TLDR: One country with the same population, size and resources but with a better fiscal system will have an easier time winning wars. Countries that were successful during this military revolution were those able to concentrate the fiscal powers.
Note: The three main gunpowder empires are: the Ottoman in Anatolia, the Mughal in India and the Safavid in Persia. They managed to conquer huge areas because their neighbours were not able to adapt to this new style of warfare.
Charles Tilly, in his book ‘’The Formation of National States in Western Europe’’ came with a theory saying that although states made the wars, it is the warfare that made the states. To put it simply, it’s like Darwinism applied to the financial and political aspects of a society. The reason why all states are so similar to this day is mostly because others have failed to collect taxes efficiently. The states wage wars and for that, they need money. Those who can get that money have more chance of winning over their enemies. Eventually, smaller countries and those with inefficient fiscal systems will tend to disappear from the map.
How can a country attain an ‘’efficient fiscal system’’? Simple, he needs to be able to collect the money were it is…
Michel Fortmann in the book ‘’Les cycles de Mars’’ (I think the book is only in French) explain that the introduction of the gunpowder during the Renaissance was a real military revolution. The number of conflicts per century on the European continent was around 8-15 during the end of the middle ages but will rise to over 100 in the following centuries. Spain will be in a state of war for 200 consecutive years and most other countries were at war 80% of the time. The coming of this new technology will increase the financial pressure on the states. States needed to adapt to a new style of war. Armies were changing and becoming more costly. The armies needed cannons, munitions, and many horses to get the cannons on the battlefield. It was also necessary to adapt military strategies to this new reality. Furthermore, the defensive system had to adapt and new fortresses appeared, made to resist the fire of the cannons. The small states could not afford it and those that could afford it (Spain, France…) were in serious financial precarity.
The problem of the medieval feudal system was that the fiscal powers were spread across the lords. The king had his personal holding that made about 10% of the country when he was lucky and maybe he received some money from his vassals, but all this is very limited. He needed the support of his vassals for everything he wanted to do. The problem is that individually, no one could afford to ‘’update’’ their army to the new technology. Cities in the center of the country did not see the need to increase military spending and building fortification but cities on the border were threatened but could not pay for it. The solution was to make everyone pay. The feudal system had to change for a more centralized government in order to get that money. It’s a slow process. In France, the king managed to impose new taxes on the cities. The lords could not assure the defence of the kingdom alone, so they accepted the lost of their powers. On the other side, the king had the legitimacy to take the fiscal powers away from the lords, most of it.
Note: According to the author, 2 important countries came to disappear because of their incapacity to centralize the fiscal powers: Poland and Hungary. I’m not knowledgeable enough but apparently, their political system was considered archaic and they were absorbed by other states. It is also interesting to mention the fate of the Spanish Habsburgs Empire and the United Provinces. One had an archaic system and the other was really decentralized. Both lost their Empires but managed to keep their independence. Spain lost control of Austria, the Low Countries, southern Italy and some other smaller possessions scattered in Europe. The Netherlands (this country, whatever you want to call it) , used to be a maritime superpower. The replaced the Portuguese’s for the eastern trade and their maritime power was unmatched. But the defence policy lacked cohesion.
But it goes one step further.
Representative government: With representation, people are willing to pay more.
Centralizing the fiscal powers doses not really make the system efficient. France before the revolution (Ancient Régime) was an absolutism monarchy. From a fiscal point of view, the problem was that most of the burden was on the poorest people. The clergy and the nobility paid almost nothing yet had all the riches of the country. Even if France had a centralized government at the end of the 18th century, a lot of regional differences, privileges and oddities persisted in their system. The system was inefficient as it was unable to extract a large portion of taxes. As I said earlier, the preceding century saw a lot of conflicts. The 17th was a period of intense warfare starting with the Austrian war of succession if I remember correctly. For France, these wars had a great cost but gave little benefits: they lost New France during the 7 years war. England was able to match France military spending despite being 3 times less populous. Part of this difference comes from the trading revenues of England but most of it comes from the higher tax burden on the English people. Even the nobility paid their share of taxes but not in France.
France was an autocracy and the population felt like the taxation system was unjust and indeed it was. In an absolutist monarchy, the king decides everything and doesn’t need to explain his actions. Therefore, it became impossible to increase tax revenues in France. You don’t want to tax the rich but the poor are already heavily taxed. England was a democracy (a representative government), while not perfect, the population could not blame the aristocracy for their problems since they also paid taxes. Furthermore, it is harder to blame a politician who increases taxes that it is to blame an aristocrat that don’t even contribute to the system. Politicians are accountable for their actions in a process called ‘’ the next election’’ but in time of needs, people are also more willing to increase their burden if the government is a democracy. The Thirteen colonies had the slogan: ‘’No taxes without representation’’, and I think it’s a good summary. With representation, people are willing to pay more.
Lastly: can this argument be used to explain why the central powers lost WW1? Probably yes for Austria but not so much for Germany. I lack the knowledge to confirm it but the author seems to think it might have played a role in their defeat.
Note: I’m not even talking about taxes on the revenue, this is very recent. Most taxes were on sales, import/export, land holdings. Tax on the revenue needs a more complex bureaucracy that will come later in history.
Problematic rules of succession:
When the ruler died, it was a custom in the Germanic kingdoms to separate the holdings equally between the sons, leading to the disappearance of the political entity. The closest thing I could find to call it is Gavelkind, a term used in the game Crusader Kings 2 originally forms the British Islands. Normally, we have another rule called primogeniture were the eldest inherited the kingdom but it was not always like that. The most famous case of this is when the Carolingian Empire was separated in three. The Treaty of Verdun officially ended the unity of the Empire for good. This practice ceased to exist in the region after that treaty. Another example is what happened to the Mongol Empire after the death of Genghis Khan. Although his sons continued to extend the Empire, it was made of completely different entities were each acted on their own.
I could also mention the 100 years war here. England and France fought for the French throne because they had a common lineage but did not agree on who should be on the throne. England recognized the right of women to inherit but not France. This almost destroyed the kingdom of France because other countries also took part in the conflict: Scotland, Burgundy, Castille.
Climatic problems:
A change in the climate, several bad harvests, less rain and a weaker monsoon all diminished the food production and let many peasants in a precarious state. Alone, it is rarely enough to put an end to an Empire. But if the problem persists, it will impact all aspects of societies. In these article:
http://www.scientificamerican.com/article/monsoon-climate-change-chinese http://www.climategeology.ethz.ch/publications/2010_Cheng_et_al.pdf
The authors talks about the fall of the Tang dynasty and the Mayan civilization. Both have gotten into decline in the same years and the authors said that it’s related to changes in the climate. In China the Tang are generally recognized as a period of growth and expansion. Like Europe before the Great Famine, the population probably exceeded the capacity of the land slightly before the climate started to change. A weakening in the summer monsoon meant less rain for Northern China and less food. This change might come from the weather system itself or in a change of the planet axial tilt because of a major earthquake. Now you know the other problem the Tang had to face and I’m not sure which one is worst.
Ming dynasty: It is said that the dynasty fell because one general guarding the Great Wall opened the way to the northern invaders. Well, the Empire was already in turmoil and the dynasty had already lost the Mandate of Heaven. In fact, the northern region was already dealing with rebels among the military and civilian. The emperor hanged himself before the invasion even started. It is hard to say what would have happened without the invasions. A general might have taken the title of Emperor for himself to establish a new dynasty. Ultimately, the Ming felt because of poor harvests. The invaders are not going to attack a state when he is strong. We should not give too much credit to the invaders. The great Arab conquests were preceded by epidemics and prolonged warfare between the Eastern Roman Empire and Persia.
Two big triggers would be food production and increased economic activity. When you have a technological advancement that yields higher food production per capita it allows more folks to not be engaged in that endeavor and can do things that advance the culture in one way or another.
The same is true of increased economic activity. For example, the medieval feudal system is essentially a completely static (economic) model. No one is getting rich, sure there is a huge divide between classes, but there is no middle class. Once something happens to change this, there is excess money that can be used to further the growth of the culture, scientific research, etc.
For example, at the start of the Renaissance you have better sailing ships which leads to increased contact with other cultures, some more advanced (Arabic, Chinese), some less so (the Western Hemisphere). From one you learn some stuff, from the others, you take stuff (natural resources, etc). All adds to growth of the civilization.
I am going to try to answer this question generally.
I think the first requirement for a major world or cultural transition is dissasisfaction. You need a large group of unsatisfied people to drvie the need for change. I think of the barbarians in Rome, the peasants in the French and Russian Revolutions. These people are the energy that drive the change. The larger the relative size of the group the closer the change, the more dissatisfied/gap between this population and the rest of society the greater the chance of violence.
Dissasisfaction is not enough. For anything to change the dissasisfaction must be channeled into a cause or solution. There can be multiple solutions to chose from but over time there is typically one idea that most people rally behind. This gives people a necessary vision to work towards. The powerful and affluent who benefit from the current situation will typically resist this cause until a triggering event.
Eventually, a triggering event will occur that force the society to react. It is typically a dramatic event like Martin Luther nailing the 95 thesis that comes after a long period of buildup. It forces society to either adapt or attempt to crush the change. The result of societies choices will create a new status que that will eventually lead to a new source of dissatisfaction. I believe it is a sign of a cultures maturity if it can resolve sources of dissatisfaciton without resorting to violence.
This cycle of dissatisfaction -> solution -> triggering event -> new status quo -> new source of dissatisfaction, while oversimplified, is useful for understanding historical events, and especially for generating your own cultural shifts in your worlds.
The substance of my answer has been mentioned in other answers but not given its due emphasis and so is getting lost in all the detail. The fundamental truth is that empires fail when they fail to adapt to changing conditions.
This failure to adapt could also be expressed as resistance to change. The innovations that gave the empire-builders their original advantage eventually become problematic when they no longer provide that advantage. People hang onto them long after their "use by" date thinking that what has given advantage will continue to give advantage even when that is no longer true. When emerging, an empire is differentiating itself and so change is actually embraced and encouraged during this phase; but a failing empire has become preoccupied with defending its institutions rather than reinventing them and so change is resisted.
(That, by the way, is the world we live in today: with Industry & Government feeling threatened by and resisting emerging ecofriendly changes for as long as they can. Political power is beginning to devolve to the local level where cities and counties are stepping up to address the issues being ignored or resisted at the State and Federal level.)
I hope these thoughts spark some ideas of changes that your empires could be embracing to their benefit or resisting to their undoing.
There are several things that can happen.
Part of what brought Greek (and then Roman) knowledge was the great discoveries in science made by the likes of Aristotle. This let the Greeks (and Romans) have more advanced technology. For example the Greeks invented the waterwheel, gears, plumbing, showers, and even alarm clocks. Then the Renaissance re-created many of these things, starting the series of technological revolutions that bring us to today. So discoveries can bring along huge advances, even in just society. The Internet for example, has changed society massively, because of increased communication.
The Renaissance largely happened because classical and scientfic texts were reintroduced to Europe through the crusades. So again, Discoveries (or rediscoveries) encourage societal change and advancement
When mass amounts of recorded knowledge is destroyed, then often times society will devolve. This happened when the barbarians destroyed Rome. They destroyed a lot of knowledge, but they also destroyed the central government and the center of society. Without Rome as a center, society drifted apart and became more inward looking. No longer was the Roman Emperor sending governors and taking taxes from everywhere. So nobles ruled over their towns, with a King ruling over an area. Literacy levels also fell, because Roman infrastructure wasn't heading everything. Once literacy was done, scientific accomplishments could be made as easily, so societal evolution slowed.
That being said, the Black Death is often attributed to the bringing on of the Renaissance. This happened because demand on peasants became much higher, enabling peasants to actually hired themselves out, instead of being basically slaves to their Lord. This established a free market of supply and demand, as well as promoting trade and scientific establishments.
Finally change needs to be spread to do anything. This kind of spread was caused for the Renaissance mostly be conquering armies. French armies invading Italy brought Renaissance ideas to that place. But peace allowed the Renaissance ideas to spread to France and England, so both ways work.
Your question is: "What similarities can be seen in the fall of [aforementioned] great empires? Whether natural events, social trends, or technological advancements. Also, how can one use this information in building worlds?"
I find it amazing that everyone only comes up with explanations based on war here. You go to war when you think you stand a chance. You think you stand a chance against an almighty empire when you're either:
It follows that a whole empire is very unlikely to fail unless c) is the case.
Historically, sociological inertia is very strong. Cultural, military and economic practices that have proven to work will not change, unless there is a strong push to change them. It follows that empires are only overrun or collapsing if 1. they have developed internal weaknesses to external pressure, which 2. manages to eventually successfully bring about the desired change. Sometimes that change can be adapted to, and sometimes it will eventually mean the collapse of empire.
Barbarians were actually powerless against Rome (or rather, could be regularly defeated), until Rome was marred by internal conflict which weakened the borders to the extent that trust in the system of power was failing. Infrastructural concepts of Roman urban space was failing (e.g. corruption kills water supply maintenance, which makes water supply in cities impure, cities become less habitable, so people began leaving the cities.) The middle ages had really begun before the barbarian invasions. A variety of socio-economic factors lead to internal weakening, which made new barbarian rulers feasible, as the "vision" of the old system had become increasingly unappealing.
To make it short: Don't look for war and technology as the answer to why empires are failing; they do play a part, usually in the end, but the internal cohesion, failure to adapt to internal dynamic processes (=political failures) and decline of cultural magnetism of an empire are what comes before any fall.
For building worlds, I take it that you look for factors that would make empires long-lasting: These empires would either exist in very (socio-economically) non-dynamic worlds, or would be highly adaptive to their surroundings and challenges (making them drop traditions at a moment's notice when these become inhibitors rather than agents of empire). So either the challenges to such an empire do not change (in both form and response), or the empire itself constantly changes (in both form and response to challenges).
I especially like the answers from Bokai and Abulafia, who cite Toynbee and Systems Theory respectively. I think those are the most generally powerful and descriptive.
To pull it all together, I'd recommend John Michael Greer's posts on the nature of empire. In a 3-piece series starting with http://thearchdruidreport.blogspot.com/2012/02/nature-of-empire.html, Greer applies his serious scholarship in both Toynbee's theories of history and Systems Theory, and a lot more besides (such as the work of Oswald Spengler and Joseph Tainter), and boils it down into a very well-written overview.
"Put more simply, an empire is a wealth pump, a device to enrich one nation at the expense of others. The mechanism of the pump varies from empire to empire and from age to age; the straightforward exaction of tribute that did the job for ancient Egypt, and had another vogue in the time of imperial Spain, has been replaced in most of the more recent empires by somewhat less blatant though equally effective systems of unbalanced exchange. While the mechanism varies, though, the underlying principle does not."
The third article, "The Trajectory of Empires", contains fundamental, general answers of the type the OP is, I think, looking for.
"The wealth of subject nations, in other words, is a nonrenewable resource for empires, and empires thus face the same sort of declining returns on investment as any other industry dependent on nonrenewable resources. It’s thus predictable that the most frequent response to declining returns is an exact analogue of the "drill, baby, drill" mentality so common in today’s petroleum-dependent nations. The drive to expand at all costs that dominates the foreign policy of so many empires is thus neither accidental nor a symptom of the limitless moral evil with which empires are so often credited by their foes. For an empire that’s already drained its subject nations to the point that the wealth pump is sputtering, a policy of 'invade, baby, invade' is a matter of economic necessity, and often of national survival."
Greer's insightful treatment of empires as complex systems gives us a pretty good grounding in what's going on. Essentially, empires exist as parasites on external sources of wealth. As parasites, they never manage to avoid the exhaustion and destruction of those sources of wealth.
Meanwhile, the empire has generated costly internal complexities in order to administer the imperial economy and society, and has also become dependent on unequal systems of economic exchange that enforce the empire's advantages. This in turn requires big military expenditures. Systems theory assures us that these layerings of complexity on complexity yield diminishing returns, so the whole edifice becomes increasingly fragile...
Why do empires fall? Fundamentally, for two reasons: exhaustion of indispensable resources, and the inherent structural progress towards diminishing returns on internal investment.
Please note that I'm not dismissing the other answers. All of them are at least partially true, I think. But I agree with Greer: the fundamental nature of an empire is the root of its own demise.
By the way: Greer's work in general is an excellent resource for worldbuilding. You may find it offputting, because he's a clergyman (he really is an Archdruid) or because he believes that our civilization is entering into a long period of descent; but his insights in military history, human ecology, systems theory, and reality-based economics are all very solid; and they amount to considerably more than the sum of the parts.
Actually, the primary impetus for a culture to advance technologically is... other cultures.
If we consider Europe, it has a very convoluted coastline and a large mountain range right in the middle, plus a lot of big rivers, which makes for a lot of different national groups, each of which in a way could be considered to be other cultures, all in a small area. Constant contact with these other cultures has led to warfare, or preparation for warfare, and warfare is the second greatest impetus for technological advancement.
Then consider other groups. China advanced rapidly when it was at war with its neighbors, but stagnated once it was considered too powerful to attack. Japan followed the same pattern, advancing when in conflict with other nations, but stagnating once they became isolationist (despite many civil wars), and then advancing rapidly after Europeans forced their way in, even though they were not at war with the Europeans. The world's most primitive groups, the pacific islanders and the Australian aboriginals, had very little contact with any other cultures until Europeans made contact and started colonizing.
In 1984, Professor Alexander Demandt published a survey of the reasons historians have given for the fall of Rome. [1,2] The list numbers 210 reasons, ranging from lead and mercury poisoning to public baths and polytheism. Rome's alleged killers are as varied a barbarians, bureaucrates, feminists, hedonists and militarists.
Personally I lean toward the systems thinking school, which states that it's a fallacy to assume that large effects have large causes. The monumental event that was JFK's assasination could have been the act of a lone loser, it does not need to be a conspiracy of proportional size. Similarly, if we could trace the cause of the fall of Rome, there may have been one or a multitude of events too insignificant to remember, that could yet have turned events on their head. Maybe a chance meeting between an influential backer and a now forgotten senator, that could have become an oustanding emperor? Maybe some years of slightly worse harvests in Germany would prevent it amassing the barbarian army in the Teutoburg forest? Perhaps some minor innovation in early Rome, which gets multiplied as its culture spreads over the centuries, to make the provinces more profitable or robust to changes in climate.
That Rome lays in the distant past does not excuse the many fanciful explanations. Everyone reading this answer were alive during the 2008 financial crisis. We were there as it happened, but the theories as to why it happened keeps increasing. It has been explained through a commodities boom, a peak-oil scenario, the theory of scale-free networks, reactionary gender roles, weakened legislation, an error in a mathematical formula, the dot-com bubble, the postwar baby boom and a number of arguments for why "the system" is set to produce regular booms and busts without outside intervention. I can count about 40 reasons in wikipedia articles alone. Within our lifetime, the number of explanations for the financial crisis may well rival that of the fall of Rome.
As for story-writing, I would love to see someone write a story from the systems thinking perspective. Maybe the protagonist does one little thing, which through unforeseen consequences escalates until he's watching helplessly, as his civilization crumbles around him. It's a tantalizing idea, that the seed of a glorious future or the end of civilization as we know it could be buried in the small and mundane around us.
Or perhaps the real literary wonder is not found in explaining the fall of empires, but in how they sustain themselves for centuries. Unlikely islands of orderliness in a sea of chaos.
[1] http://www.wired.com/2013/06/210-reasons-for-the-fall-of-the-roman-empire/
TLDR: The question is to broad to answer in anything less than several books
This question is like asking what one or two things makes people die or get sick, except empires of hundreds of humans are more complicated than individual ones.
This question encompasses basically the entire field of history like the second encompasses the whole field of medicine.
Its an important question but there are hundreds of different causes, we can point to empires that had popular unrest for decades before they fell(like running a fever), and to ones that fell suddenly and violently to outside invasion (gunshot wound). You can't point to a single symptom to say this is what caused every person to die like wise you can't point to one cause for the fall of empires. (aside from birth: statistically most people have been born died and the rest are likely to die in the next 100 or so years, most empires that have been founded have fallen)
Worse still there are many empires where the cause is highly debated or the result of many different problems which further slows analysis.
In fact the largest collapses seem to be the most murky and complex
The right answer would have to be a list of hundreds of possible causes ranging from loss of irreplaceable leaders (Macedonia) Combat with a more powerful foe (Japanese empire in the 1940's)
To more murky causes hyperinflation from gathering to much sliver and gold (Spanish Empire) increased reliance on mercenaries/ decadence and waste of the upper class / dozens of important causes I'm forgetting (Rome) Loss of trade route control due to better ships
To make things even worse some major empires we have almost no information on why they fell, like the Mayan and Mohenjo-daro civilizations
I will try to answer in the context of the "Civilization" game.
Empires get unwieldly, and vulnerable when they are "too large." Too many people in a city makes some of them unhappy. Too many cities in an empire reduces the necessary number of people in any given city before it becomes unhappy. Cities (and military units) that are unhappy are susceptible to revolutions from within, and bribes from without. Also, the further a city is from the capital, the higher the levels of corruption and waste (relative to its size).
In playing the game, I consciously do NOT try to maximize population growth. For instance, I do not irrigate land until relatively late in the game (e.g. when I have a government like Democracy that fights corruption and waste). Instead I use my settlers to build roads that connect my cities, thus reducing corruption and waste. I also tend to place workers on "resource," rather than "farmland" squares, which further slows population growth, but speed up the construction of buildings. Temples and colosseums calm restive people, courthouses reduce corruption, marketplaces and banks boost the economy etc. Basically, I want a high ratio of buildings (capital) to people.
If I capture small cities abroad, I "depopulate" them by building settlers until the city disappears, using military units for "rush builds." Thus, I get a "free" settler or engineer to improve my other cities nearer home.
The collapse of the ancient civilizations occurred when then got so "top heavy" that 1) other civilizations feared and envied them and 2), they were not "agile" enough to resist outside threats. Instead, managed growth for internal, rather than external growth seems like the better bet. But most rulers didn't see it that way until it was too late. Louis XV's quote, "After me, the flood," preceded Marie Antoinette's "Let them eat cake."
