The first major expansion of the Achaemenid Persian Empire took place under Cyrus the Great in the middle of the sixth century BCE, and it was he who established several of the systems and policies which were to allow the empire to flourish and expand further in the two centuries that followed.
Sometimes the policies pursued by Cyrus were adapted as the situation changed. For instance, Cyrus preferred to leave the local ruler and infrastructure in place after conquering a region. This allowed him to spend minimal time and resources on administration before moving on to new conquests. He allowed the people to continue to practice their traditional religion and only insisted on the regular payment of tribute.
However, this system quickly altered to the establishment of regional satrapies. The satraps were viceroys, reporting directly to the high king and presiding over regions where they paid attention to local customs and religions, as the former rulers had. To ensure that the powerful satraps did not get out of hand, however, an official known as the "eye of the king" performed yearly inspections.
Under both the original rulers and the satraps, Persian administration was comparatively tolerant and unobtrusive, giving local populations little reason to rebel. Some groups—such as the Jews, who were allowed to return to Israel by Cyrus after being driven into exile by the Babylonians—had good reason to be loyal to Persia.
The Persians were renowned horsemen and made more extensive use of horses, both for conquest and administration, than any previous empire. An extensive program of road-building began under Cyrus, and the main transport artery, the Royal Road, was completed by Darius. The journey from Susa to Sardis, some 2,699 kilometres, could be completed on horseback in nine days. This provided a safe passage for traders as well as a regular flow of information to the king (and eventually a more extensive postal service). Darius also standardized the currency, producing a gold coin called the "daric" which could be used throughout the empire. This promoted trade and travel between provinces and further assisted the social and economic cohesion of the empire.