In the 1st half of the first millennium BCE, the Iranian plateau was occupied by Mede, Persian, and Parthian groups, all of which were Indo-European Iranian sub-types. These three groups were under the subjugation of the Neo-Assyrian empire, especially in the western end of their country, but the Medes, sensing weakness, campaigned to end their subjugation (and eventually the Neo-Assyrian empire itself).

The Medes were centered in the west of modern Iran, their hub being the city of Ecbatana. Under their king Cyaxares, they assaulted Nineveh in 612 BCE and ended the Neo-Assyrian domination. At this time, the other Iranian groups were weaker and soon came under the domination or direct influence of the Median state.

The Persians, in the south of modern Iran and centered around the urban centers of Pasargadae and Persepolis, formed a sort of client kingdom within the Median state. This changed soon, however, when the Persian Cyrus the Great led a revolt against the Medes and ascended the throne in 550 BCE. This began the Achaemenid Persian state/empire.

The third Iranian group, the Parthians, would get their shot in the wake of Alexander's destruction of Achaemenid power. Parthia was a region in the northeast of modern Iran, abutting what is now Turkmenistan. The Parthians would dominate the region in the Hellenistic and Roman periods, only to be eventually supplanted by the Sassanian Persians again the the 3rd century CE.