A Social Animal
Humanity, because it is a social animal, lives in groups known as societies. These are shared understandings, founded upon shared beliefs about right and wrong, expressed in a shared language, which allows society to behave as a single creature. But a creature with a mind whose memory is unlimited by biology, so constantly accumulates the lessons taught by the experience of its members; thereby becoming increasingly wiser and more powerful.
Societies Are Incompatible
As different societies have incompatible beliefs about right and wrong, each will strive to violently impose their understanding upon every other society. In this way a victorious society will impose its beliefs upon all other surrounding societies, and so form an empire, or civilisation. This is why war is an inherent part of humanity.
Respect For Elders Essential
However, like all creatures, a society has a lifespan, which is the length of time a society can maintain its reverence for its founding beliefs about right and wrong. These are maintained and expressed as traditions, which are the sanity of society. And this sanity is refined, along with its traditions, as the shared mind improves its understanding. However, this process depends upon each generation revering their elders, for when this reverence is lost, society loses its shared understanding and becomes senile.
Signs Of Senility
The first sign of social senility is society's discarding of its colonies, as it has lost the vigour needed to maintain them. Following this public display of weakness, society is then invaded by unarmed barbarians seeking to profit from this feebleness. An act that must eventually win the eradication of that society, along with its civilisation.
The Recurring Cycle
This is why the history of humanity is the recurring cycle of the rise then fall of civilisations, separated by Dark Ages: a time when there is no dominant society.