This dissertation examines how Greek authors from the fifth to the second century BCE employed the concept of pleonexia to explain why cities lost power on the international stage and why they lost internal cohesion. First, it argues that Greek authors understood pleonexia to mean "the desire for more at the expense of another" as opposed simply "greed" as most modern authors translate it. Second, it contends that Greeks authors deployed the concept of pleonexia to describe situations that modern authors would describe as societal collapse--defined as the reduction of societal complexity, which can be measured through either the loss of material or immaterial means, e.g., land, wealth, political power, influence over others, political stability, or political autonomy. Greek authors used the language of pleonexia to characterize the motivation of an entity, either an individual within a community or a city or state, to act in a way that empowered the entity by taking or somehow depriving another similar entity of wealth, land, or power. In a city, pleonexia manifested as an individual seeking to gain power through discrediting, prosecuting, or eliminating rivals. In international affairs, it materialized as attempts of a power to gain more territory or influence over others. Acting on such an impulse led to conflict within cities and in the international arena. The inevitable result of such conflict was the pleonexic power losing more than it had had before. The Greeks, thus, had a theory that acting on pleonexia led to a reduction in societal complexity. Tracing this paradigm in over two hundred years of Greek writing further demonstrates continuity in Greek thought across the Classical and Hellenistic cultural boundaries imposed by modern writers. The dissertation thus argues that Greek authors used pleonexia to construct a psychological model of decline that persisted for over two hundred years.