Arsaces, the leader of the Parni, had been watching the chaos in Parthia closely. He knew that Andragoras was isolated and weak. In 247 BC, Arsaces led his warriors across the border. These weren't ordinary soldiers; they were 'Cataphracts' and horse archers who could shoot deadly arrows even while retreatingโa skill that would later baffle Roman armies.
The invasion was swift and brutal. Andragoras, who had betrayed his own emperor, found no allies to help him. Arsaces defeated and killed him, seizing the region of Parthia for himself. To legitimize his rule, Arsaces crowned himself king at Asaak, where an eternal fire was lit to honor the new dynasty.
The Seleucid Emperor Seleucus II eventually tried to retake the lost province, but it was too late. The Parni had adopted the name 'Parthians' and fortified their positions in Hyrcania. They used the rugged terrain and their mobile cavalry to outmaneuver the heavy Greek phalanxes. The Parthian victory was complete, and 247 BC was celebrated forever as the beginning of the Arsacid Era.