The kingdom that Mithridates inherited in 165 BC was one of the many medium-sized powers that had risen with the decline of Seleucid Empire or had appeared on its borders. Other kingdoms were
Greco-Bactria,
Cappadocia,
Media Atropatene, and
Armenia. Mithridates I's domains encompassed present-day
Khorasan Province,
Hyrcania,
northern Iran, and the southern part of present-day
Turkmenistan.
Wars in the east He first turned his sights on the Greco-Bactrian Kingdom which had been considerably weakened as a result of its wars against the neighbouring
Sogdians,
Drangianans and
Indians. Mithridates I afterwards appointed his brother
Bagasis as the governor of the area. This victory was followed by the Parthian conquest of Media Atropatene. In 141 BC, Mithridates I captured
Babylonia in
Mesopotamia, where he had coins minted at
Seleucia and held an official
investiture ceremony. There Mithridates I appears to have introduced a parade of the
New Year festival in
Babylon, by which a
statue of the ancient Mesopotamian god
Marduk was led along parade way from the
Esagila temple by holding the hands of the goddess
Ishtar. With Mesopotamia now in Parthian hands, the administrative focus of the empire relocated towards there instead of eastern Iran. Mithridates I shortly afterwards retired to Hyrcania, whilst his forces subdued the kingdoms of
Elymais and
Characene and occupied
Susa. of
Demetrius II Nicator,
basileus of the
Seleucid Empire Whereas
Hecatompylos had served as the first Parthian capital, Mithridates I established royal residences at Seleucia, Ecbatana,
Ctesiphon and his newly founded city, Mithradatkert (
Nisa), where the tombs of the Arsacid kings were built and maintained. Ecbatana became the main summertime residence for the Arsacid royalty — the same city which had served as the capital of the
Medes and as summer capital of the Achaemenid Empire. Mithridates I may have made Ctesiphon the new capital of his enlarged empire. The Seleucids were unable to retaliate immediately as general
Diodotus Tryphon led a rebellion at the capital
Antioch in 142 BC. However, an opportunity for counter-invasion arose for the Seleucids in when Mithridates I was forced to leave for the east to contain an invasion by the
Saka. The Seleucid ruler
Demetrius II Nicator was at first successful in his efforts to reconquer Babylonia, however, the Seleucids were eventually defeated and Demetrius himself was captured by Parthian forces in 138 BC. He was afterwards paraded in front of the Greeks of Media and Mesopotamia with the intention of making them to accept Parthian rule. Afterwards, Mithridates I had Demetrius sent to one of his palaces in Hyrcania. There Mithridates I treated his captive with great hospitality; he even married his daughter
Rhodogune to Demetrius. According to Justin, Mithridates I had plans for Syria, and planned to use Demetrius as his instrument against the new Seleucid ruler
Antiochus VII Sidetes (). His marriage to Rhodogune was in reality an attempt by Mithridates I to incorporate the Seleucid lands into the expanding Parthian realm. Mithridates I then punished the Parthian vassal kingdom of Elymais for aiding the Seleucids–he invaded the region once more and captured two of their major cities. Around the same period, Mithridates I conquered the southwestern Iranian region of
Persis and installed
Wadfradad II as its
frataraka; he granted him more autonomy, most likely in an effort to maintain healthy relations with Persis as the Parthian Empire was under constant conflict with the Saka, Seleucids, and the
Mesenians. He was seemingly the first Parthian monarch to have an influence on the affairs of Persis. The coinage of Wadfradad II shows influence from the coins minted under Mithridates I. Mithridates I died in , and was succeeded by his son
Phraates II. == Coinage and Imperial ideology ==