The more you have, the more you have to lose. At some point the cost of holding your territory becomes greater than the benefits you gain from that territory.
The question is rather why do they function as long as they do. I refer to Robert A. Heinlein’s succinct summary of the nature of human society:
'Throughout history, poverty is the normal condition of man. Advances which permit this norm to be exceeded- here and there, now and then- are the work of an extremely small minority, frequently despised, often condemned, and almost always opposed by all right-thinking people. Whenever this tiny minority is kept from creating, or (as sometimes happens) is driven out of a society, the people then slip back into abject poverty.This is known as “bad luck.” ’
Personally, I suspect that in any large organization of people the inevitable realization occurs that the efficiency of collective action can be significantly improved if it doesn’t have to go through the messy process of persuasion – if people can simply be* told* what to do.
And, indeed, that works for a while. The trains really can be made to run on time, if El Supremo – or the Senate, Committee on Public Safety, Federal Reserve governor, whatever – says they must and wields the power of life and death over everyone involved.
However, in such a society it is inestimably better to be one of the tellers than the told, so…your class of tellers and directors and organizers and managers and secretaries and viziers and governors – your entire parasitic class, which produces nothing but commands and reports – quite naturally grows without limit, as everyone seeks to become part of it. Eventually the sheer weight of the parasites cannot be supported on the dwindling backs of the productive. When productive men would rather be free than rich, you have reached the end, and things will collapse.
BTW, if you are considering the Roman Empire in particular, be sure to look at the role of the tax burden on the merchant and farmer class, and of debasement of the currency (inflation) by the Imperial Court. Keep in mind the small landowners in the West willingly adopted feudalism – they *preferred * vassalage to Roman citizenship. That tells you a lot about what it was like to be an honest Roman citizen in the late Empire.
If empire doesn’t work to the advantage of the governed - and, most of the time, it doesn’t - then it’s inherently unstable. So the question is not what causes empires to collapse, but what keeps them from collapsing?
The colonies becoming independent and separating themselves from the host government. Ex. Spain, France, Great Britain all at one time had global empires but eventually those broke off.
In general, I would say failure to evolve or adapt.
The British, Spanish, and Roman Empires, like many others, grew with a model that required continuing growth. When they hit the limits - to big, too far to the frontier, no room for expansion - that model stops working. The center feeds on the growing inputs from the outside states. The costs of maintaining these states become higher as they become more settled, more populous, more restive - but being internal territories, paying off the troops (and the center) with plunder becomes a zero-sum game; actually negative, since destroying your own holdings is counter-productive.
Russia, for example, was substantially poorer than some of its East European vassals because of the cost of maintaining and upgrading the army to hold those states. Gorbachev made the decision that it was not necessary spending any more - at which point they, and then Soviet internal holdings, split off.
The USA successfully converted a westward expansion economy into an industrial sales economy, becoming industrial supplier to the world. This allowed it (and much of the rest of the west) to finance an increasing social safety net model and an greater level of infrastructure for the country. However, bit by bit this model is being chipped away - cloth and clothing, which was one of the drivers of the British and American initial growth, has been underbid by other countries. The same is happening to each industry in turn - steel, shipbuilding, autos, etc. So far, the USA is the leader still in higher tech - aircraft, computers, space, entertainment, military, internet technology.
The question is - how long can it keep working like this, and what is the next model? We already see some European countries hitting the same wall, their tech industry cannot pay for the level of citizen services they want to provide.
On the one hand, you have the imperial power and its people. When they started out, they were presumably relatively weak and poor and were looking to get stronger and richer. They were willing to go out and take risks and fight wars. And their early successes gave them a hunger for more.
But at some point, the rewards of their empire sunk in. The people that had been weak and poor realized they were now strong and rich. They began thinking more about how much they had to lose and less willing to fight for more.
Meanwhile, on the other hand, you have the subject people. Back when they were conquered they were poor and weak (otherwise they wouldn’t have been conquered). But as time goes by and they’re part of the Empire, they learn from their conquerors. Even as a subject people, they grow richer and stronger. And at some point, they decide they’re now rich and strong enough that they should no longer be a subject people.
I don’t think this is true at least for the British Empire. It was still a vibrant empire until it hit the grinder of WWI and was significantly weakened. WW II was the final blow.
This may attach in some way to Heinlein’s construction, but some would suggest that a main cause of collapse is the increased entrenchment of the vested interests. At some point, profiteers, politicians, and the military (not mutually exclusive concepts) may lose sight of the needs of society as their own interests take precedence, leading ultimately to collapse.
Maybe.
I think it was hitting the wall, the new territories (central Africa, Australia, Canada etc.) were settled to the point they could. As the locals became more productive and rich, they resented enriching the central country. The existing territories were starting to become problematic and require extra babysitting (the Boer War). Britain avoided a number of these problems by spinning off the trusted (white) and less problematic territories as almost co-equals: Canada, Australia, South Africa. But the writing was on the wall, regarding cost of far-flung empire maintenance vs. benefits derived. I suppose the WWI costs simply hurried the failure, just as WWII simply delayed the issues with various European empires - India, French Indochina, Indonesia, central Africa, etc.