Diodotus I Soter (Greek: , Diódotos Sōtḗr; c. 315/300 BCE – c. 235 BCE) was the first Hellenistic king of Bactria. Diodotus was initially satrap of Bactria, but became independent of the Seleucid empire around 255 or 245 BCE, establishing the Diodotid Greco-Bactrian Kingdom, which endured in various forms until the beginning of the first century CE. In about 250 BCE, Diodotus repelled a Parthian invasion of Bactria by Arsaces. He also minted an extensive coinage and administered a powerful and prosperous new kingdom. He died around 235 BCE and was succeeded by his son Diodotus II.
His rule was recounted by Apollodorus of Artemita in the Parthian History, but this text is lost, and surviving literary sources only mention him in passing. Thus, most details of Diodotus' life have to be reconstructed from numismatics and brief references by Justin. and other historians.
Background and satrapy
The region of Bactria, which encompassed the Oxus river Valley in modern Afghanistan and Tajikistan, was conquered by Alexander between 329 and 327 BCE and he settled a number of his veterans in the region. In the wars which followed Alexander's death in 323 BCE, the region was largely left to its own devices, but it was incorporated in the Seleucid empire by Seleucus I between 308 and 305 BCE, along with the rest of the territories that Alexander had conquered in Iran and Central Asia. Seleucus entrusted the region to his son and co-regent, Antiochus I, around 295 BCE. Between 295 and 281 BCE, Antiochus I established firm Seleucid control over the region. The region was divided into a number of satrapies (provinces), of which Bactria was one. Antiochus founded or refounded a number of cities on the Greek model in the region and he opened a number of mints to produce coinage on the Attic weight standard. After Antiochus I succeeded his father as ruler of the Seleucid empire in 281 BCE, he entrusted the east to his own son, Antiochus II who remained in this position until he in turn succeeded to the throne in 261 BCE.
Diodotus became Seleucid satrap (governor) of Bactria during Antiochus II's reign. The Babylonian Astronomical Diaries record that an unnamed Bactrian satrap sent a herd of twenty war elephants to Babylon at the beginning of 273 BCE to join the Seleucid forces fighting against Ptolemaic Egypt in the First Syrian War. This satrap may have been Diodotus, or a predecessor. Archaeological evidence for the period comes largely from excavations of the city of Ai-Khanoum, where this period saw the expansion of irrigation networks, the construction and expansion of civic buildings, and some military activity, probably raiding by nomads from the Central Asian steppe. As satrap, Diodotus was probably involved in these matters, though the specifics are not recoverable.
Secession from the Seleucid realm
At some point, Diodotus seceded from the Seleucid empire, establishing his realm as an independent kingdom, known in modern scholarship as the Graeco-Bactrian kingdom. The event is mentioned briefly by the Roman historian Justin:
The date of this event is unclear. The literary evidence is as follows: Justin says that the rebellion occurred 'around the same time' as the Parni conquest of Parthia from the Seleucid realm, but his dating of this event is confused—he places it in 256 BCE, but during the reign of Seleucus II (246-225 BCE). Strabo further claims that Arsaces, the leader of the Parni, had been based in Bactria before the conquest. He says that Diodotus drove Arsaces out of Bactria and maintained hostilities against the Parni. Ammianus Marcellinus places the Parthian rebellion in the reign of a Seleucus (II?). Arrian's lost Parthian History seems to have claimed that the Seleucid satrap who was overthrown by the Parthians was appointed to that position by Antiochus II. Appian states that the Parthian rebellion took place in 246 BCE, during the Third Syrian War, in the wake of Ptolemy III's conquest of Seleucid Syria and Babylon. The Adulis inscription set up by Ptolemy III to celebrate this event claims that Bactria was among Ptolemy's conquests, which is hyperbole but might indicate that Bactria had been part of the Seleucid empire up to this point.