The Achaemenid Empire (558–330 BC) of Persia, popularly referred to as the Persian empire, was a monarchy. It was ruled by a single hereditary leader, who considered himself divinely authorized to hold absolute power.
The Persian empire was a model of efficient ancient administration. The monarch appointed satraps as regional leaders, and delegated power in a way the preserved sufficient local autonomy to prevent most (non-Greek) subjects from wishing to revolt. It had an efficient system of roads and messengers, allowing rule over a large geographic area, and a regular system of taxation that established it on a sound financial footing. It also had a complex and uniform law code.
Persians, like other Iranian groups, formed a tribal confederacy. Each of these tribes inhabited a certain part of Persia, and their territories were well defined. Their social formation does not seem to be much different from their Indo-European ancestors, being a basic patriarchal system based on several blood related families forming a tribe, and the tribes ultimately forming the confederacy. There seems to have been a firm oligarchic system in place in which the heads of tribes would make all large decisions regarding the general conduct of the society. These Elders belonged to an upper level of the society, the “ruling class”, whose members held the chief positions by the right of birth.
Membership in other social classes - clergy, artisans, herdsmen – was also hereditary, although Iranian tribes in general seem to have avoided the experience of their Indian cousins in creating an untouchable class, mostly consisting of the natives. Persians, as well as Medians and Parthians, easily married with the local population, whom they probably served initially as mercenaries and herdsmen when they first arrived in the plateau