At the time, the Achaemenid Persian empire was the largest empire that had ever existed. Without modern means of rapid mass communication, it was impossible to control such an empire directly. The Persians built a network of roads and waystations, and installed satraps (regional governors) and garrisons throughout the empire, but even so, large parts of their domain were beyond the immediate reach of the Royal Court. To keep these cities and regions loyal, it was essential to secure the cooperation of the local population.

The Persian empire - like many other empires in history - therefore made frequent use of a power relationship in which the conquering Persian King would magnanimously allow defeated communities to retain their own religious and political customs and traditions in exchange for submission to fixed obligations such as tribute payment and military service. In other words, they allowed parts of their empire to more or less run themselves in whatever way they pleased, as long as they promised to serve the King and to pursue no independent foreign policy.

We should not mistake this for an ideology of tolerance. The likely origin of Carlin's claim is the so-called Cyrus Cylinder, a document which affirms the right of the conquered Babylonians to worship their own gods and retain the existing institutions of the city. This document is often cited as the earliest expression of the idea of universal human rights, but in fact it is little more than one side of a bargain struck between conqueror and conquered. In return for his generosity to the Babylonians (and to the Jews held in Babylon, whom he allowed to return to Palestine), Cyrus the Great would have expected loyalty, tribute and soldiers to add to his imperial might.

We see this process at work in Persia's relations with the Greek cities of Western Asia Minor. Conquered as part of the Lydian kingdom by Cyrus the Great in 546 BC, these cities were subjected to tribute, as well as being made to contribute ships to the Persian fleet. However, the cities were not run directly by the satrap established at Sardis. Instead, the Persians propped up indigenous (that is, Greek) tyrants, who ran their communities on the Persians' behalf, secured in their local supremacy by the guarantee of Persian backing as long as they kept the tribute flowing to the Persian capital.

It so happened that (for reasons we don't quite understand) the tyrants of the Ionian cities gradually became very unpopular. In 499 BC, the tyrant of Miletos, who had begun to fear for his own position for complicated reasons, united most of the region in revolt against Persia, and the first thing the cities did in order to assert their independence from Persia was to cast out their tyrants and install democratic systems of government.

It took the Persians 5 years to crush the so-called Ionian Revolt, but by 494 BC they had defeated the Greeks' combined armies and fleet and regained complete control of the region. At that point it was up to them to reassert their control over the cities in whatever way they saw fit, and we might expect them to abolish the new democracies, reinstall the pro-Persian tyrants, and impose an even heavier tribute. But in fact, as Herodotos tells us, they allowed the Ionian cities to keep their democracies; the king's cousin Mardonios was sent to reassess the tribute through careful measuring of each city's land and resources, and the result of his survey was that the former tribute level was deemed fair.

What we're seeing here is a clear example of a (violent) redefinition of the terms by which a particular region was subject to Persia. Since the population had shown itself unhappy with the existing arrangement of rule through tyrants, the Persians did not deny them their democracies; in order to avoid further unrest, they did not increase the tribute. The result of their reassessment of the terms of subjection was that Ionia and the surrounding regions reverted to loyal service and reliable tribute payment until they were liberated/conquered by the Athenians in the 470s BC.

Again, the lenience shown by the Persians was not the result of some professed ideology of tolerance and altruism. It was, to some extent, a calculated attempt to find a balance between the interests of Persia and the need to ensure the willingness of their subjects to accept Persian rule. This occasionally meant intervening in local political systems to guarantee a local regime's loyalty, but on other occasions it meant acknowledging the wishes of the local population in order to secure that same loyalty. The network of unequal settlements between Persia and its subjects, expressed in such things as the Cyrus Cylinder and the arrangement with the Ionian cities, allowed the Persians to maintain a happy ideological fiction in which all the peoples of the Empire were cheerfully united in the task of supporting the rule of Ahura Mazda's representative on Earth.