What are the elements that give way as a civilization declines into oblivion, such that centuries are required for recovery? First, let us begin by noting that civilization doesn't fall all at once because knowledge is destroyed. Barbarians don't destroy cities because they hate civilization. They conquer them because they want their share of the wealth, and books are valuable items. Indeed, they often contain the knowledge of how wealth is built. So, if a library burns, it was likely an accident, or if not, it was someone's stupidity.
But civilizations do fall, sometimes so slowly that no one sees the decline, and no one sounds an alarm. They break down one step at a time, as critical elements fall into disuse, or hit a snag that prevents a function for a season. When people learn to get by without some hard-to-get item, they forget why they needed it. Hardship can become normal in a decade or two.
These are the steps by which previous civilizations declined. Some even fell so far that their legends were forgotten.
1. Foreign trade in essential supplies
2. Skills for producing utilitarian needs
3. Security for trade economics
4. Monumental architecture
5. Bureaucratic record keeping
6. Currency production
7. Education in culture
8. Glory lost beyond memory
Civilization is vulnerable whenever keystone essential skills are dependent upon certain trade goods that can be cut off, or when they are under contract to a single payer or limited customer base. If demand ceases, and no one wants the product of a skill, no one will practice it, and no one will teach a new generation of artisans. Then all of the observed tacit knowledge that isn't written down but only shared in practice will be lost.
There is never a decision to abandon a capability, but only a year or two in which there are other needs more important than continuing it in production. This goes on just long enough for the people who understood how to do it move into other jobs, or die without passing on their knowledge. Then, if there isn't enough basic infrastructure to reconstruct the process from analyzing old products, there will be no one capable of continuing it. Future generations will have to rebuild it from scratch.
Each generation looks at the current level of capabilities and establishes it as their recognized baseline. Whether it goes up or down, the capacity will be measured against that baseline. Thus, the decline, or rise, does not seem as dramatic up close as it would if seen from a longer perspective, but rises or falls on a gradual scale of about thirty years.
The recovery also must proceed in stages, regaining each element in order. Each generation pulls itself up, one step at a time, trying to build on hopeful imagination for greater things yet to arise in a more glorious future. But always, what they now experience in the field before them places limits upon what they imagine to be currently possible.
1. Rudimentary consumer production
2. Substitutions for lost supplies
3. Centralized authority
4. Acceptable currencies
5. Foreign exchange
6. Long distance security for trade
7. Educated classes
8. Demand for sophisticated products.
There is rarely possible any great leap forward without strong leadership imposing a drastic reorganization for a new vision and drawing capabilities into a new focus. This will inevitably wait for the governing authority to perceive that opportunity exists for a monumental achievement. In the meantime, society must regather itself gradually.
The real danger of a prolonged dark age arises when there is no one who remembers that we were ever able to do such a thing in the past, and any records of it having been done come to be regarded as legend or fantasy. Even visible ruins can come to be regarded as the work of long dead giants. At that point, centuries can roll by without anyone dreaming of rising beyond their daily mundane experience.
Remember, a dark age rarely happens because some barbarian conqueror smashes and burns a city holding a precious library. It doesn't happen because knowledge is destroyed. Rather, it happens because no one wants to buy a specialized product made by a skilled artisan. It gets neglected this year, then for a decade or so, because there are more pressing matters that need to be attended to. Maybe, the next time it's recalled to mind is by someone's grandfather reminiscing about his glory days, and by then it's too late. No one remembers how it was done. Then, in a few more generations, no one bothers to tell their children that the world used to be more wonderful. The memories will be gone.
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