How Did Ancient Empires Work?

I have long wondered what it was about the Inca and Aztec empires that allowed small groups of Conquistadores to replace their ruling classes relatively quickly. I have been reading Persian Fire: The First World Empire and the Battle for the West by Tom Holland, and it raises similar questions in my mind. The book discusses the Greek-Persian wars of the fifth century BCE. Cyrus the Great established the Persian empire in the sixth century BCE,as the Persians conquered the lands of the Medes, Babylon, Lydia, etc. Darius the Great succeeded Cyrus' son, coming from a different tribe within the empire, and subduing a number of revolts. Not long thereafter Alexander the Great conquered the Persian empire. The question in my mind is how did relatively small groups gain control of relatively large populations scattered over large geographic areas?

Of course, one aspect is that there was enough agricultural productivity to support a ruling class large enough to run a large empire. Holland gives some added clues. There was an extensive set of Persian roads that allowed rapid movement across the empire. There were food warehouses scattered over the empire so that troops moving from place to place could be supplied from those warehouses. There was a system of rapid communications, including post riders traveling the road system and beacon fires, allowing (for the time) very rapid communication over long distances.

I wonder to what degree these elements were based on social and technological infrastructures developed over hundreds of years prior to the Persian empire? Is it merely coincidence that the empire developed to rule the fertile crescent of antiquity?

Certainly the military advantage of the new rulers played a role. The Persians had excellent cavalry as well as heavy and light infantry and archers. The military genius of Alexander is well known.

I think there had to be advantages to belonging to empires. Merchants could benefit from trade advantages by being within an empire (I suppose not only some protection of trade routes, but also some elements of common law and standards). Local political factions could gain power by affiliation with imperial powers.

And of course, the sanctions imposed on cities that resisted conquest or revolted against the empire could be draconian -- including mass slaughter and raising of the city.

The political culture of the time must be taken into account. It was a time of clan and tribal alliances and of city states. Factions contested for power within the cities. They would sometimes (often) sell out the city to an invader for their own political advantage. Intercity alliances were fragile, and commanders would abandon their allies and leave the battlefield, especially if given the right incentives by the enemy.

The imperial governments tended to be masters of Machiavellian manipulations. They bribed factions within governments and broke coalitions of opposing cities. The imperial governments also manipulated local religious beliefs, suggesting that local gods of conquered peoples must have favored their conquests or that local gods of peoples who had unsuccessfully revolted must be false gods.

Ultimately, I think, the culture of the time was so different from our own that it is hard to empathize with what happened. We don't really think of California or New York deciding to not send troops to Iraq; indeed, even if a NATO nation decides to withdraw troops from Afghanistan, it does so in an orderly fashion with advance warning. The coups, espionage, sabotage, and other machinations of factions within the city states seem far from the cultural experience of modern Western states.

On the other hand, Iraq is still a society with strong tribal affiliations. Saddam Husein was called al-Tikriti for the city from which he and his family came, an indication of how important the local culture remains within Iraq. There are also of the differences between Kurds and Arabs, Shiites and Sunnis. Indeed, I wonder whether leaders from the United States and Europe were predisposed by our Western culture to underestimate the cultural and thus political complexity of Iraqi society and to misunderstand the imperatives for governance of such a society.