Over the years, most nominees for the Libertarian Futurist Society’s Prometheus awards have been science fiction, and set in the present or the near or far future. Harry Turtledove’s Between the Rivers is an exception: A work of fantasy, and set in an invented world that parallels the Bronze Age of 3000 years ago or more.
As its title suggests, its setting is based on Mesopotamia. It’s a realm of contending city-states, and seems to be the first place in its larger world to develop them; at any rate, there’s no indication of comparably civilized realms elsewhere.