Seleucid–Parthian Wars participant Parthian Empire
Background
In 323 BC, the Seleucid Empire was founded by Seleucus I Nicator, a general of Alexander the Great. Stretching from Syria to the Indus River and comprising most of Alexander's realm, the Seleucid state was the most powerful of the Diadochi kingdoms that sprang up after Alexander's death. Quickly however, the Seleucids ran into trouble trying to maintain such an extended realm, facing constant warfare against the other Hellenistic states in the west and with unrest amongst their Iranian peoples in the east.
Taking advantage of the Seleucids' preoccupation with the wars against a Celtic invasion of Asia Minor in the west, and the chaos of the Third Syrian War around 245 BC, Diodotus and Andragoras, the Seleucid satraps of Bactria and Parthia respectively, declared their remote provinces independent states. However, in around 238 BCE, the Parni, an Iranian tribe from the central Asian steppes under Arsaces, invaded Andragoras' domain, defeated and killed him, and took over the land.