What causes the fall of civilizations?
There are many possible explanations for this. Let us examine two of the more important ones.
First, there is the idea that civilizations fall because they consume all of their resources and create environmental catastrophes that make them fall. This idea is notably set forth in Jared Diamond's book Collapse. It has also been used to explain the fall of civilizations like those of Mesopotamia and Greece.
Second, there is the idea that large empires (which could be called civilizations) fall because they overreach themselves and become too large and expensive. This idea can be seen in Paul Kennedy's book The Rise and Fall of the Great Powers. According to this, civilizations grow and grow and try to extend themselves. They end up becoming too big and too expensive and they cause their own downfall in that way. This explanation is also sometimes used to explain why Rome fell.