re: Historical Regressive Societies

The best example, in my opinion, was the Visigoths. During their siege of Rome, they destroyed the aquaducts supplying the city with water. After conquering the city and slaughtering people, there was no one left who knew how to repair the aquaducts. Without that source pf water, the city's remaining freshwater supply could only support 1% of the population it had prior to the siege. In the span of a century, Rome's population dropped from 1 million people (already only half as populous as its peak prior to the capital moving to Constantinople) to only around 10,000. There simply weren't enough knowledgeable people or resources left to maintain the infrastructure anymore, and no standing army or security force to protect traders and travelors on the roads or keep the open.