MITHRADATES I (Arsaces VI.), successor of his brother, Phraates I., came to the Parthian throne about 175 B.C. The first event of his reign was a war with Eucratides of Bactria, who tried to create a great Greek empire in the East. At last, when Eucratides had been murdered by his son about 150, Mithradates was able to occupy some districts on the border of Bactria and to conquer Arachosia (Kandahar) ; he is even said to have crossed the Indus (Justin 41, 6; Strabo xi. 515, 517; cf. Orosius v. 4, 16; Diod. 33, 18). Meanwhile the Seleucid kingdom was torn by internal dissensions, fostered by Roman intrigues. Phraates I. had already conquered eastern Media, about Rhagae (Rai), and subjected the Mardi on the border of the Caspian (Justin 41, 5; Isidor. Charac. 7). Mithradates I. conquered the rest of Media and advanced towards the Zagros chains and the Babylonian plain.. In a war against the Elymaeans (in Susiana) he took the Greek town Seleucia on the Hedyphon, and forced their king to become a vassal of the Parthians (Justin 41, 6; Strabo xv. 744). About 141 he must have become master of Babylonia. By Diodorus 33, 18 he is praised as a mild ruler; and the fact that from 140 he takes on his coins the epithet Philhellen ( W. Wroth, Catalogue of the Coins of Parthia, p. 14 seq. ; till then he only calls himself " the great king Arsakes ") shows that he tried to conciliate his Greek subjects. The Greeks, however, induced Demetrius II. Nicator to come to their deliverance, although he was much pressec in Syria by the pretender Diodotus Tryphon. At first he was victorious, but in 138 he was defeated. Mithradates settled him with a royal household in Hyrcania and gave him his daughter Rhodogune in marriage (Justin 36, i, 38, 9; Jos. Ant. 13 5, n; Euseb. Chron. I. 257; Appian Syr. 67). Shortly afterwards Mithradates I. died, and was succeeded by his son Phraates II. He was the real founder of the Arsacid Empire.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)