Arsaces, who was chief of the Parni, a member tribe of the Dahae confederation, must have begun his struggle against the Seleucids from 247 BC, the year from which the Parthians dated their history. This does not necessarily mean that Arsaces was crowned king in 247. Other Iranian dynasties (e.g., the Sasanids; see below The Sasanian period) dated the beginning of their eras from the time when they began to establish their power rather than from the time of coronation of the first monarch of their line.
Daho-Parno-Parthian tribes “chose chiefs for war and princes for peace” from among the closest circle of the princely family. They were famous for their breeding of horses, for their combat cavalry, and for their fine archers. Alexander encountered them during his Bactrian campaign, and the Greek writers who recorded his reign remarked on their agility and effectiveness as horsemen. They were a people who kept the traditions of patriarchal tribal organization. The Parni, with Arsaces at their head, took the province of Parthia after having beaten Andragoras; soon, neighbouring Hyrcania was annexed and the Caspian reached. Arsaces had himself crowned in the city of Asaak, and the tribe took the name of the Parthians, their close relatives, a name that meant “exiled.” Their language was closely related to Scythian and Median. The dynasty these people produced never broke its links with the people, and rare was the Arsacid dynastic sovereign who did not turn to his people in time of danger.