To a large extent, the Persian Empire incorporated the region of Parthia simply because it was there. Parthia is situated squarely on what is now much of northeastern Iran, and its northern border is defined by the Kopet Dag mountain range, which provided a natural frontier, and a vast desert region to its south. Its location between those two natural, and forbidding, formations made it a part of the East-West trade route that develop into the Silk Road, extending from modern-day Turkey to China. In its strategically important location, the Parthians were able to leverage their geographic advantage to emerge as a major trading center between China and the Roman Empire. Its geographic characteristics also made it an important acquisition by the Persians, led by Cyrus the Great, whose conquest of Parthia made his subsequent imperial ambitions possible by providing strategic depth essential to protect his westward-bound armies. Also, Parthia’s western border ran along the southeastern coast of the Caspian Sea, an important route for naval movements along the northern flank of Media, on which Cyrus had set his sights. While greatness lie ahead for Parthia, during the era of the Persian and Roman Empires, it was, besides its location along the major trade route for Eurasia, little more than a colonial backwater. The following is an observation of Parthia from an ancient Roman history attributed to Pompeius Trogus, as adapted by the historian Justin:
“During the time of the Assyrians and Medes, they [the Parthians] were the most obscure of all the people of the east. Subsequently, too, when the empire of the east was transferred from the Medes to the Persians, they were but as a herd without a name, and fell under the power of the stronger.” [See “A Roman Description of the Parthians or later Persians from Justin’s History of the World,” www.shsu.edu/~his_ncp/Parthian.html]
To reiterate, Parthia was important because of its proximity to the major East-West trade route connecting Europe and Asia, and because its conquest was important to the broader territorial ambitions of the Persian Empire. For the most part, however, it was incorporated into the empire simply because it was there. The defeat of the Persian Empire by Alexander the Great, however, would pave the way for the eventual rise of the Parthian Empire.