This is where the theory of diminishing returns begins to take effect. As an empire expands and gains more and more “capital” (i.e. money, resources, etc.), the sociopolitical complexity of that polity will naturally become more complex. For example, the Aztec conquests throughout Central Mexico resulted in the massive accumulation of material goods throughout the Valley of Mexico, especially in Tenochtitlan. This led to a population explosion, with the Valley of Mexico’s population increasing three to five fold over the course of 200 years.
The increased population required more resources in order to sustain, especially as the elite began to expand their influence throughout the empire. Even if conquest is halted, complexity will continue to increase so long as material goods are brought to the imperial center. Based on this model, continuous conquest is needed in order to sustain the empire.
The ability of an empire to continuously conquer territory is reliant upon a variety of factors. As a universal rule, however, target regions which are far away from the imperial center are far more expensive to conquer than regions which are closer. In order to conquer regions outside of the immediate periphery, expensive logistical structures with the ability to support long distance campaigns for extended periods of time are necessary.
Eventually, as an empire begins running out of territory to conquer, long distance campaigns become far too expensive and conquest no longer generates enough profit. This is known as the point of “diminishing returns”, when the investment no longer makes profit, but breaks even instead. When the investment reaches the point of maximum yield, the expenses exceed the profits and sustainability becomes impossible.
This is how many empires begin to go into decline. When imperial polities reach their maximum potential territory, the cost of sociopolitical complexity will continue to increase, and gradually cause the decline of the entire civilization.
The Incans were better
Bumpin'