At the start of 165 BC, Phraates attacked the powerful
Mardians (also known as Amardians), a group of people who lived in
Alborz mountains, which bordered
Hyrcania in the east and
Media in the southwest. Owing to their geographical position, the Mardians were able to pose a threat to the trade routes stretching from Hyrcania and western Parthia to western Iran. The attack was probably part of the Parthian efforts to expand their domain in Iran proper and secure control over Hyrcania. The main aspiration of the Parthians was to conquer Media, starting with Media Rhagiane. Phraates' attack on the Mardians was successful, conquering the Caspian Gates, as well as the city of
Charax, which was close to the Median metropolis of
Rhaga. Furthermore, he also reclaimed Hyrcania from the Seleucids. He had a group of Mardians deported to Charax to protect the Caspian Gates, and the
Tapurians in Parthia deported to the
Caspian coast, which gave rise to the name of the historical region
Tabaristan. Phraates' conquests paved the way for his successors to further expand the Parthian realm. Phraates' western expansion was a transgression of the traditional
status quo between the Parthians and Seleucids. During this period, the Seleucids
were occupied in
Judea, which suggests that Phraates deliberately mounted his campaign during a time where the Seleucids were unable to respond. The Seleucid king
Antiochus IV () left Judea to prepare to mount a retaliation campaign against the Parthians, but died near
Gabae at the end of 164 BC, probably due to disease. His successor, the nine-year-old
Antiochus V Eupator () was unable to focus on the Parthians, as his reign was marked by conflict, political intrigue, and
Roman influence. Phraates notably appointed his brother Mithridates as his successor. It was common amongst
Central Asian nomads for a ruler to be succeeded by his brother instead of his son. This practice may have survived amongst the Arsacids, owing to their nomadic origins. A passage by the 2nd-century Roman historian
Justin suggests that Priapatius had chosen Mithridates as the successor of Phraates. Olbrycht supports this theory, stating that Phraates was by himself not in a position to choose his brother over his sons, due to his short reign. Justin reports that the interests of the country was of higher importance to Phraates I than that of his sons, which indicates that he supported the decision made by his father regarding the succession. Phraates I died in 165 or 164 BC, and was succeeded by Mithridates I. == Coinage ==