Civilisation is the blossoming then shrivelling of human understanding, manifested as the rise then fall of a dominant society.
A Dominant Society imposes its beliefs upon all other societies by violence (war) or its threat, which must involve the threat, or use of, genocide so any society that recoils from inflicting genocide will suffer genocide.
Its Empire of Colonies: All those societies that survive by submitting to the demands of the dominant society insensibly adopt some of the traditions of their new ruler, so becoming its colonies; and together these colonies form its empire.
A Society is a shared understanding; a creature that has a lifespan set by its ability to pass its shared understanding onto successive generations. This demands imbuing each generation with its founding beliefs about right and wrong, which are the foundation of every understanding, and form its religion. Inevitably each new generation will contain a number of unbelievers, who eventually become the majority. Whereupon the shared understanding is lost and the society declines. Hence a society:
Waxes to a prime under the influence of a majority of citizens who revere a shared religion where:
Common Purpose is revealed so uniting the citizens and energising the society.
Replaces superstition with science so winning increasing wealth and power
Refines its traditions so clarifying the duty of citizens needed to improve their society, thus refining manners, purpose and identity, thereby winning an increasingly ordered and disciplined society.
Wanes under the influence of a majority of citizens who revere only themselves and their opinions, where:
Common purpose is lost so stagnating and disuniting the society
The discarding of tradition as unnecessary restraints so discarding duty along with the manners, identity and purpose of citizens needed to maintain social order thus transforming the previously ordered and disciplined society into a mob, so corrupting all its institutions
N.B. As a civilisation is a society, and a society is an understanding, then the study of civilisation must be the study of understanding, which is the realm of philosophy.