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.
Blossom Then Shrivell
A society blossoms for as long as citizens revere tradition, for this is the sanity and source of vigour for the society. With the strength of this reverence being revealed by the size of church congregations. When the churches are full, the society reveres tradition so has vigour and sanity. But when congregations start to shrink the society is losing its reverence for tradition, thereby losing its sanity and vigour, so shrivelling into senility.
Signs Of Senility
The first major 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.