“Astyages had a daughter called Mandane, and he dreamed one night that she urinated in such enormous quantities that it filled his cities and swamped the whole of Asia,” writes Herodotus in his history of the Greek war with Persia. Astyages was the king of the Median Empire which at its peak controlled about 2.8 million square kilometers from western Anatolia to nearly the Indus River in modern Pakistan.
The king’s magi interpreted the dream as a prophecy that Mandane’s son should one day be called king. Although she was of royal rank, Astyages sought to avoid the birth of a son of royal class by marrying her to Cambyses, a man from a good family in Persia—which was then under Median dominion—yet of a much lesser rank than she. When Mandane had a son, Astyages, fearing overthrow, handed the child over to one of his kinsmen with orders to have the child killed.
Of course, as happens in these stories, the surviving child, Cyrus, rose to power and established the Persian (or Achaemenid) Empire at around 500 BCE. He first conquered the Median Empire and rapidly spread west annexing Mesopotamia, Egypt, the rest of Anatolia, and eastern Thrace. To the east, he expanded south to the Arabian Sea and north into Scythian territory. At its peak the Persian Empire controlled over 5.5 million square kilometers, the largest empire that had ever existed.
Empires of that size have been of obvious interest to historians since Herodotus and his contemporaries. They’re where all the action is. Warfare and political intrigue make for great books in any century, and the actions of empires have created the modern world as we know it. From 3000 BCE to 1800 CE, there were at least 60 mega-empires which controlled over a million square kilometers. Empires pose a problem for social scientists as well. The development of large-scale cooperative political organizations is one of the enduring puzzles in the social sciences. Humans tend to act much more prosocially than is expected from evolutionary genetic models, forming cooperative groups far larger than should be expected from our ability to track social information and past records of cooperation and betrayal.
The group sizes of most primates tends to hit a ceiling at around the point, we think, where it becomes too challenging for an individual to track the relationships between not only an individual and others, but those between others and others. As group size increases, the number of such relationships increases at a high enough rate that eventually the groups end up fissioning into smaller groups. Humans have a greater ability than other primates to track social information. However, since the invention of agriculture, group sizes have skyrocketed. In modern times, our cities can be populated by millions, and the populations of states can be as much as a billion.
Our unique ability to form these large coalitions and societies composed of largely unrelated individuals with shared goals—peace, stability, subsistence—pursued by shared effort is difficult to explain. Unlike most other species, we share food with each other—and not just with our kin. We are also willing to put a lot of effort into the active defense of our groups, even to the death. Some social scientists in the last decade have begun to sift through the historical data—both written and archaeological—to develop models that might help explain the patterns in history, in this case, why massive political organizations can develop. In 2009, Peter Turchin of the University of Connecticut published an essay in the Journal of Global History which argued that the formation of mega-empires is the logical conclusion to an arms race between neighboring agrarian groups living in fertile environments and pastoralist groups that live in arid steppe regions.
He sketches the scenario like this. In regions with a sharp gradient between high and low rainfall, the people living in the fertile region take up an agrarian form of subsistence and those living in the arid regions take up a pastoralist form of subsistence. Pastoralists generally have a higher initial military advantage, particularly if they have horses which were domesticated in the steppe regions of Eurasia just before the string of large empires began to form. Pastoralists have more hunting and riding experience than the agrarians, which are useful in military campaigns. They also have a motivation to raid the agrarians, whose stationary concentration of grain and luxury goods make them an attractive target.
Turchin explains that there are three ways in which the agrarian societies can increase their defensive abilities against the pastoralists. The first is that they can create voluntary defensive pacts with neighboring communities that are also raided by the pastoralists. This is potentially effective, but the agreement is also vulnerable to freeriding, in which one group accepts defensive help from other groups but does not help to defend others. Second, one agrarian community can annex their neighbors and force them into a mutual defense arrangement, with the possibility of punishment if help is refused. The third possibility is that some members of the pastoralists can invade and rule a set of agrarian communities. They would move into the agrarian communities and become an elite, assimilating into the local culture and speaking the local language but maintaining a foreign born elite.
Any of these three possibilities would create a pressure for the pastoralists to politically organize as well. The process then takes off as the pastoralists and the agrarians scale-up into larger regional political entities continuously raiding and invading each other. Eventually, a limit will be hit in which the empires become large enough that it becomes impossible to effectively rule at a distance. Cultural and technological innovations can often serve to extend these limits. Many social scientists have argued that the organized religion developed as one such cultural tool with which large political organizations could remain cohesive. Technological developments such as the chariot (~2000 BCE) also appeared during the same period and in the same places as the formation of these large empires.
This pattern is seen in the development of most of the mega-empires throughout history, summarized well in Turchin’s article and most likely in one or more of his books, though I haven’t read any of his books yet. The areas surrounding the arid belt that spans Afroeurasia from the Sahara Desert in northern Africa to the Gobi in northern China has spawned the majority of the sixty mega-empires (table on page 202 and map on page 204 of Turchin’s article), with the exceptions of the Khmer Empire in Southeast Asia, the Inca Empire in South America, and the Roman and Carolingian Empires in Europe.
In addition, although one might expect that the political centers of an empire would be positioned near its geographical or economic center, the political centers of most empires are positioned near the agrarian/pastoralist frontier where the original impetus to politically organize was located. For example, the capitals most of the many empires in China were located to the north, though most of China’s economic strength was gradually shifted to the more fertile south. Turchin argues that this was because Beijing was nearer to the raiding pastoralist federations such as the Mongols, the Turks, and the Xiongnu.
Likewise, although Egypt’s cultural and economic power was in the north nearer to the Nile delta, all of the great Egyptian unifications originated in the south near the frontier with the Nubians, who were largely pastoralists. When the rainfall levels to the Sahara declined the Nubians and other pastoralist raiding groups such as the Libyans were forced to live further from Egyptian territories. This was a military relief to Egypt, yet they were never able to establish another empire.
Turchin covers several other regions in his article—Eastern Europe, the Maghrib in Northwest Africa, the Middle East, Southeast Asia, and the Great Plains of North America—and he covers those that I’ve described here in far greater detail. This type of historical inquiry is refreshing an promising. It will not be able to account for the unfolding of particular historical events, yet as it advances, patterns of why certain types of events tend to happen at certain times and in certain places will help us understand the historical development of our species. It will also give us a template with which we can compare the present to the past to avoid the types of conflicts that have occurred in the same conditions for millenia.