This aspect of history -- what is it that causes civilizations to fail? -- is one that has fascinated me for years. The best answer I've seen yet came from a Georgetown University professor named Carroll Quigley.
Quigley's analysis (in his wonderful book, The Evolution of Civilizations) followed in particular the works of Spengler and Toynbee. For both Spengler and Toynbee, civilizations emerged, matured, decayed, and died, in that order and always.
As Quigley looked at Western civilization, however, he concluded that Western civilization didn't fit that one-way pattern, but had somehow recovered from challenges similar to those that had destroyed earlier civilizations. In fact, the West reinvented itself not just once, but twice.
To Quigley, this was undeniable evidence that Spengler and Toynbee, while right in many things, were wrong in this one critical thing: the decline and fall of a civilization is not inevitable.
From this, and from the related data of history, Quigley surmised the following:
1. Civilizations progress through roughly seven stages:
1. Mixture2. Civilizations grow (in territory, population, wealth, and knowledge) as long as they have an "instrument of expansion." Examples of instruments are religious tribute (Mesopotamian civilization), political tribute (Egyptian), slavery (Classical), feudalism (Western), and capitalism (the West again). In each case the instrument allowed the civilization to invent (to produce a surplus beyond what's needed for survival), to save (to concentrate that surplus in a few hands so that it has more effect), and to invest (to apply the surplus to additional invention).
4. Age of Conflict
5. Universal Empire
3. But all instruments institutionalize -- eventually, the special interests in whose hands the surplus is concentrated reduce their level of investing in further invention and begin retaining the surplus for themselves. As the public becomes discontented by a decline in the rate of expansion, the civilization enters the period of imperialist wars and popular irrationality that Quigley characterized as an Age of Conflict.
4. Unlike Spengler and Toynbee, Quigley believed history demonstrates that it's possible to recover from the institutionalization of an instrument of expansion, that civilizations can return to an age of Expansion. He concluded that there are two ways to do this: by reforming the broken institution back into a working instrument, or by circumventing the old institution with a new instrument of expansion. In either case, the civilization returns to the Expansion phase.
5. Civilizations that fail to reform or circumvent the institutionalization of their instrument of expansion proceed to becoming a Universal Empire. This appears to be a golden age, but only because the civilization's resources are being spent in a final burst of consumption. Once this happens, there's no going back -- that civilization will inevitably proceed to decay and be invaded, and its cultural assets will be divided among its conquerors.
All of which is probably way more than anybody wanted to know. But now we can look at the world of the Federation with an eye toward what could make it enter an Age of Conflict.
To do that, we first need to consider: What was the Federation's instrument of expansion? And second, who controls that instrument, and why would they begin to keep for themselves the surplus it creates?
My off-the-cuff guess for the instrument that allowed the Federation to expand would have to be "warp drive." I think replicator technology would actually have more of an impact in the real world, but in the world of Star Trek it's warp drive that has enabled the access to resources and cultures that enabled the Federation's remarkable expansion through the Alpha Quadrant. (Many Star Trek: Enterprise episodes seem to support the idea that effective warp drive actually allowed the creation of the Federation itself.)
So who in the Federation controls warp drive? Simple: Starfleet. But why would the people running Starfleet (and remember, these aren't just humans) start thinking of the rewards of warp drive as finite, and therefore that they only "win" by accumulating those rewards to themselves instead of reinvesting them back into invention that benefits the whole Federation? What limits to additional warp drive-fueled expansion has the Federation finally encountered?
I have some ideas there, but I'll leave these questions as an exercise for anyone who's managed to read all this. I will suggest, however, that a threat to warp drive capability is the kind of thing that could pose a grave and long-term danger to the Federation. As such, it could work well in driving the ongoing story action in a persistent-world Star Trek MMORPG.