The Seleucid king Antiochus I Soter (280-261) was the first to take measures. It is certain that he refounded a city in Margiana; up till then, it had been known as Alexandria (because it was founded by Alexander the Great in 328), but from now on, it was to be called Antiochia. This military settlement was intended to guard Iran against incursions from nomad tribes, such as the Parni.
It was no sufficient, however. In 245, the satrap of Parthia, a man named Andragoras, revolted from the young Seleucid king Seleucus II, who had just succeeded to the throne. In the confusion, the Parni attacked and seized the northern part of Parthia, a district known as Astavene, probably in 238. About 235, a Parnian prince with the name Tiridates (modern persian : Tirdad meaning 'Great archer') ventured further south and seized the rest of Parthia. An counter-offensive by king Seleucus ended in disaster, and Hyrcania was also subdued by the Parni.
From now on, the Parni were known as Parthians. In the years that followed, their kings recognized the Seleucid king as their superiors, but under Mithradates I(171-138 BCE) they conquered Media, Babylonia, and Elam. The Parthian empire was to last until 224 CE, when it was succeeded by the Sassanid empire.