Parthia was led by the Arsacid dynasty, who reunited and ruled over the Iranian plateau, taking over the eastern provinces of the Greek Seleucid Empire, beginning in the late 3rd century BCE, and intermittently controlled Mesopotamia between ca 150 BCE and 224 CE.
Parthia (mostly due to their invention of heavy cavalry) was the arch-enemy of the Roman Empire in the east; and it limited Rome's expansion beyond Cappadocia (central Anatolia).
In 41 BC Parthia, led by Labienus, invaded Syria, Cilicia, and Caria and attacked Phrygia in Asia Minor.
It was inhabited by an Iranian tribe, the Parthava of the inscriptions of Darius; the correct Greek form is HapOvaioc.
Parthia became a province of the Achaemenian and then of the Macedonian Empire.
But Seleucus was soon recalled by a rebellion in Syria, and Arsaces returned victorious to Parthia; " the day of this victory is celebrated by the Parthians as the beginning of their independence " (Justin xli.