The
Parthian Kingdom started in 238 BC with Arsakes I. But it must be remembered that it started small and slowly expanded. Arsakes II was even obligated to submit to
Seleukid authority under Antiochos the Great, who was defeated by the
Romans in 190 BC.
It was under Mithradates I (171-138 BC) that the
Parthian territory was greatly expanded. According to
Shore, Mithradates I defeated the king of Media in 148 BC. Elam was defeated after 141 BC, and
Parthia established control over Elam and Persis at this time. Mithradates' I gains were not consolidated until the rule of Mithradates II (123-88 BC).
By pre-Arsacid, means before the
Parthians dominated Persis and not before the establishment of the
Parthian Kingdom. So the pre-Arsacid period is some time before 140 BC or so, with the
Parthians expanded into Persis.