After Alexander the Great's death, Persia fell for the most part to Seleucus I and his successors (the Seleucids), but their grasp on the vast territories was weak administratively, although they did introduce a vital Hellenistic culture, mingling Greek with Persian elements. Media Atropatene (see Azerbaijan) was never really under Seleucid rule. The rulers of Bactria from the beginning were at least quasi-independent and in the middle of the 3d cent. revolted and established absolute independence.
At the same time Parthia under the leadership of the Arsacids (see under Arsaces) cast off Seleucid rule and established a Parthian empire as a sort of successor to the old Persian Empire. Although even under the greatest of the Parthians (Tiridates, Mithradates I, and Mithradates II) the realm did not have the old extent, it was formidable and was a rival to Rome. The Romans in almost continuous warfare failed to halt the Parthian drives to the west, which were often supported by local ambitious or frightened rulers under Rome. Only in the 2d cent. A.D. did the Parthian rule begin to wane.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.