During the reigns of Seleucus I (312–281 BC) and Antiochus I Soter (281–261) the Parni (Aparni) nomads probably moved from Central Asia into Parthia and seem to have adopted the speech of the Parthians and been absorbed into the settled population.
Incorporating Greek infrastructure and administrators, Seleucus adopted Persia's form of governance. With districts or satrapies headed by satraps (governors) beholden to a central government and ultimately the king, Parthia became one of those satrapies.
In 245 BC, a satrap named Andragoras revolted from the young Seleucid king Seleucus II, who had just succeeded to the throne. In the confusion, Parthia was overrun by the Parni, a nomad tribe from the Central-Asian steppe.
In about 250 BC., they launched an invasion under their leader Arsaces. Known as the Parthians after their successful conquest of the land, they made their own imperial aspirations clear by instituting a dynastic era in 247 B.C., and subsequent rulers assumed the name Arsaces as a royal title.
Arsaces I was a governor under Diodotus, king of the Bactrian Greeks, and then revolted before fled westward to establish his own rule.
This small independent kingdom, they rose to power under king Mithradates the Great (171-138 BC). The Parthian empire occupied all of modern Iran, Iraq and Armenia, parts of Turkey, Georgia, Azerbaijan, Turkmenistan, Afghanistan and Tajikistan, and -for brief periods- territories in Pakistan, Syria, Lebanon, Israel and Palestine. The empire lasted for nearly 500 years and consisted of multiple cultures, which influenced the course of the empire and helped contribute to its prosperity.
In 113 AD, the Roman Emperor Trajan made eastern conquests and the defeat of Parthia a strategic priority, and successfully overran the Parthian capital, Ctesiphon. Parthian power evaporated when Ardashir I, ruler of Istakhr in Persis, revolted against the Arsacids and killed their last ruler, Artabanus IV, in 224 AD. Ardashir established the Sasanian Empire.
The origin of Parthian Empire
