Phraates I of Parthia (Parthian:𐭐𐭓𐭇𐭕 (Frahāt), Persian:فرهاد (Farhâd)) was ruler of the Parthian Empire from c. 176–171 BC. He subdued the Amardians (lat. Amardis), mountaineers occupying the eastern portion of the Elburz range, south of the Caspian Sea. He died relatively young, and appointed as his successor not one of his sons, but his brother Mithridates I (165–132 BC).
The exact date of his ascension is the subject of some confusion. He may have succeeded his father Phriapatius (185–170 BC) on the throne. However, recent evidence from Nisa suggests that a great-grandson of Arsaces I reigned briefly after Priapatius' death in 170. In this reconstruction, this previously unknown "Arsaces IV" reigned ca. 170-168 BC and was then succeeded by Phraates I (which would make Phraates "Arsaces V").
At the beginning of his reign, Phraates I invaded the territory inhabited by the Amardians, a poor but warlike people, who appear to have occupied the eastern portion of the Elburz range, south of the Caspian Sea, what is probably today immediately south of Māzandarān and Astarabad. The reduction of these fierce mountaineers is likely to have occupied him for some years, since their country was exceedingly strong and difficult.
Although, at that time, the Amardi were nominally subjects of the Seleucids, we don't see any mention of assistance being rendered to them, nor even any complaint being sent by Seleucus IV against unprovoked Parthian aggression.
An explanation for this passivity of Seleucus IV Philopator, who was characterized as weak and pacific, might be war exhaustion in Syria in this period, a consequence of his father Antiochus III's great war against Rome (192–188 BC) and the heavy contribution which was imposed upon the Seleucids by the Treaty of Apamea. Syria might scarcely have recovered sufficient military strength to enter upon a new struggle, especially with a remote and powerful enemy. Seleucus IV also may have thought that the material interests of the Seleucid Empire were only marginally affected by Parthian aggression, since the Amardi were too poor to provide much tribute, so Syria considered their subjection rather a formality than a fact. Therefore, he allowed reduction of the Amardians, probably conceiving that their transfer under Arsacid dominance would neither increase Parthian power nor diminish his own.
As Phraates faced no resistance from the Seleucids when he conquered the Amardians, he resolved to append additional adjacent territory to his kingdom. This was the tract lying immediately to the West of the Caspian Gates, which was traditionally assigned to Media (Assyria), forming a distinct district known as Media Rhagiana. It was naturally a very fertile region, being watered by numerous mountain streams originating in Elburz range, and possessing a soil of remarkable productiveness. Its breadth was not great, since it consisted of a mere strip between the mountains and the Great Salt Desert which occupies the whole centre of the Iranian plateau, but it extended in length at least a hundred and fifty miles, from the Caspian Gates to the vicinity of Qazwin. Since remote antiquity its capital city was Rhages, situated near the eastern extremity of the strip, probably at the spot now called Kaleh Erij, about twenty-three miles from the Gates. A full conquest is doubtful, but at a minimum he established lodgment in its eastern extremity, which put the whole region in jeopardy.
Parthian aggression was checked as long as Rhages remained in Seleucid possession, and Rhagiana, the rest of Media, and the other provinces were safe. Conversely, the loss of it to Parthia would lose the rest of Rhagiana, which hadn't any other natural protection, as well as the adjacent eastern provinces. After Phraates surmounted the Gates and established a foothold in the plains beyond them, he resettled a portion of the Amardians from their mountain homes into the city of Charax, west of the Gates, probably on the site now occupied by the ruins known as Uewanikif. This stronghold threatened neighboring Rhages, which could scarcely endure against enemy encamped at its doors.
We are not informed, however, of any results which followed on the occupation of Charax during the lifetime of Phraates. As his reign was a short one, it is probable that he died before there was time for his second important conquest to have any further consequences.
Phraates had sufficient warning of his coming demise to make preparations with respect to a successor. Though he had several sons, some of whom were (we must suppose) of sufficient age to have ascended the throne, he left his crown to his brother, Mithridates. He probably believed that his kingdom required the leadership of a firm ruler who could repel either Syrian or Bactrian aggression at any time; in addition, he also trusted Mithridates better than any of his sons to conduct aggressive expeditions with combined vigor and forethought, should Parthia pursue the path of conquest which it entered upon during his reign.
It also appears that Phraates bore special affection toward Mithridates, since he used the name of “Philadelphus” (transl. brother-loving) upon his coins.
Phraates I’s nephew Phraates II was initially passed over for succession presumably because of his immaturity, which wasn’t suitable, as the country was struggling with famine. Later, however, Mithridates would appoint Phraates II as his successor. The appointment was nevertheless in accordance with Mithridates I’s vision of a king that would strengthen the empire. Particularly strong was his desire to conquer Media and the rest of the Haron fortress.