Every completed civilization known to historians has passed through a sociological life cycle: origin, growth, expansion, maturity, violent premature dismemberment and death in the competitive survival struggle or gradual decline and eventual dissolution.
Every completed civilization has had small, local beginnings, on an island like Crete, or a group of islands like the Japanese Archipelago, or a tiny spot like Latium on the Tiber River, or an isolated area like the desert-surrounded Nile River Valley in Africa. The seed ground or nucleus of each civilization has been a small, well-knit group of vigorous, energetic people, well-led, living in an easily defended, limited area, enjoying relative isolation, but also having ready access to the outside world.
At the beginning the growth cycle has moved slowly, from victory to victory, as competing neighboring peoples have been brought under the authority of the victor in local wars. After generations or centuries of struggle a point is reached at which the nucleus of the growing empire begins to expand, through trade, colonization, diplomatic alliances, conquest, into an era of survival struggle in which rival cities reach out for the same piece of fertile land, the same markets, the same mineral deposits. Again the life and death survival struggle tests out the people, their leaders, their ambitions, determination, tenacity.
Earlier struggles were local. Now the struggle area has become regional. At the outset the peoples were amateurs in the science and art of expansion, occupation, consolidation, exploitation. Through the hard school of struggle they became professionals. From victory to victory they gained in territory, in wealth, in administrative skill. One by one, rivals were eliminated, annexed or associated with the nascent empire which was by way of becoming the central empire of a maturing civilization.
Generations of effort and centuries of time have gone into the empire building process. The farther the civilization has expanded, the greater the necessary input of manpower, wealth, enterprise and administrative talent needed to keep the enterprise strong, solvent, masterful.
Eventually the expanding civilization reaches a point at which the costs of further expansion are greater than the income derived from further extension of its authority. Up to this point expansion had paid its own way. Beyond this point it is a losing proposition--politically, economically, sociologically. At this point begin times of troubles; bad harvests; colonial or provincial revolts; power struggles between individuals or classes in the homeland; new rivals moving in to share in the prospective plunder of the mother-city.