Strabo, Geography

But when revolutions were attempted by the countries outside the Taurus, because of the fact that the kings of Syria and Media, who were in possession also of these countries, were busily engaged with others, those who had been entrusted with their government first caused the revolt of Bactriana and of all the country near it, I mean Euthydemus and his followers; and then Arsaces, a Scythian, with some of the Däae (I mean the Aparnians, as they were called, nomads who lived along the Ochus), invaded Parthia and conquered it. Now at the outset Arsaces was weak, being continually at war with those who had been deprived by him of their territory, both he himself and his successors, but later they grew so strong, always taking the neighboring territory, through successes in warfare, that finally they established themselves as lords of the whole of the country inside the Euphrates. And they also took a part of Bactriana, having forced the Scythians, and still earlier Eucratides and his followers, to yield to them; and at the present time they rule over so much land and so many tribes that in the size of their empire they have become, in a way, rivals of the Romans. The cause of this is their mode of life, and also their customs, which contain much that is barbarian and Scythian in character, though more that is conducive to hegemony and success in war.