Why Was the Parthian Empire So Powerful?
Today, most people in the West know little about the Parthian Empire and if they do it is in reference to the Parthians being the enemies of Rome. The Parthians and Romans were eternal enemies for much of Rome’s late Republic and early Imperial phases, with the Parthians often inflicting humiliating defeats on the Romans. The Parthians defeated the Roman general Crassus at Carrhae in 53 BC, Mark Antony in 36 BC, and took several Roman eagle standard over the course of their many wars. Eventually, though, the Romans were able to decisively defeat the Parthians in the late second century AD, but by that time the Parthian Empire had been in a long state of decline.
The Parthian Empire was established in the third century BC and lasted until the third century AD, making it one of the longest enduring empires and dynasties in world history. In terms of geographic scope, the Parthian Empire stretched from Bactria (present day Afghanistan) in the east to the Euphrates River in the west and comprised scores of different ethnicities, languages, and religions. The Parthians dominated the Near East and became Rome’s rivals through sound economic decisions and a long-standing martial background. The Parthians grew rich by controlling the legendary Silk Road and they used their wealth to build a state of the art army that used tactics that were novel to western armies.
The Background of the Parthians
The men who ruled the Parthian Empire were descended from an ethnic group known as the Parni, which is where the people and the region of Parthia in Persia received their names. The Parni originally hailed from the eastern shores of the Caspian Sea and later migrated west after Alexander the Great’s invasions into the region during the fourth century BC.  The Parni spoke a northern dialect of the Middle Persian language, but the rulers of the Parthian Dynasty, sometimes referred to as the “Arsacids” for the first king of the dynasty, Arsaces I (ruled ca. 247-217 BC), actually adopted Greek as their official language. 
After the Parthians made their way west, they settled in northern central Persia (modern day Iran). Although the Parthians spoke a Persian dialect and followed many Persian religious and cultural conventions, they were not linked directly by blood to their Achaemenid Persian predecessors. However, the Parthians viewed themselves as the legitimate inheritors of the Persian Empire and followed Achaemenid administration methods, along with several cultural attributes.  By following Achaemenid methods of imperial administration, the Parthians dealt with their large and diverse subject population by allowing them to continue to follow their religions and to generally keep their cultures as long as they paid their taxes to the crown and recognized the authority of the Parthian royal dynasty.  Another Achaemenid administrative practice that the Parthians adopted was the construction and/or rededication of several cities.
The construction of new cities was a tradition that both the Achaemenid Persians and the later Seleucid Greeks brought to Mesopotamia and the Near East. The Parthians followed in this tradition by building several cities in Mesopotamia and central Asia, including Dara, which was built by Arsaces I. Mithridates I (reigned 171-139 BC) then followed in Arsaces I’s footsteps by building the city of Mthradatket/Nisa.  These cities were minor, though, compared to Ctesiphon and Seleucia.
Although the Parthian kings, like the Achaemenids, traveled in a circuit to the major cities in their empire, essentially taking the capital city with them wherever they went, the Romans regarded Ctesiphon as the Parthian capital.  Ctesiphon was among the most important Parthian cities year round due to its location in Mesopotamia on the north bank of the Tigris River, which put it close to the border with Rome and at the end of the lucrative caravan routes. Due to these factors, Ctesiphon was eventually turned into a permanent winter capital by Pacorus, the son of Orodes II (ruled 58-38 BC).  Across the river from Ctesiphon was the Seleucid era city of Seleucia. After the Seleucid Dynasty was vanquished by the combined pressure of the Romans and Parthians, the Parthians continued to use the Hellenistic city of Seleucia, making it one of their own. During the early Parthian Dynasty, Seleucia served as an important administrative center and as a financial center, housing several royal mints.  The trade routes that ended in Ctesiphon and the coins that were minted in Seleucia and the Parthian economy, in general, were among the major reasons why the Parthians became such a powerful force in the region.
When it came to economics, the Parthians were the beneficiaries of a favorable geographic location. The Parthians were not known for being particularly excellent merchants the way other ancients peoples such as the Phoenicians and Palmyrenes were, but they controlled the lucrative caravan routes that snaked their way through central Asia, connecting the early Western and Eastern worlds. These caravan routes would collectively become known as the “Silk Road.” The most important early contacts the Parthians made were with the Chinese when in 121 BC the Emperor Wu of the Han Dynasty sent an embassy to Mithridates II (reigned 124-88 BC) to establish formal relations between the two empires. The Chinese knew the Parthians as “An-Hisi” and were particularly interested in developing an alliance with the martial-minded central Asian people and to gain access to their horses, which could be used to combat nomadic hordes.  Parthia’s trade with China had the combined effect of opening diplomatic relations with the Han Dynasty and also the emergence of the Silk Road.
Although the Silk Road had been in operation in some form before the Parthians, the Parthians established a control over the road that allowed them to collect sizable profits from the nascent silk trade, which they then used to fund their military. After the Chinese established control over the Tarim Basin around AD 90, silk merchants from northern China only had to pass through four kingdoms: China, Kushan, Parthia, and Rome. The Parthians were able to charge a hefty tax on silk passing through their kingdom from China, which was then processed in Roman territory. Interestingly, the raw silk that made its way into Rome was rewoven in factories in Syria, where it was then made into the brocade that is associated with silk today. From there the finished silk would make its way to Rome proper and ironically back east to China.  The Parthians profited twice from their position; first by charging the raw silk to pass through their kingdom to the west and then when they charged to send the finished silk back through their kingdom to the east. The caravans went through the Parthian city of Merv daily and could be quite large – a single caravan could include up to 1,000 Bactrian camels with each camel carrying 400 to 500 pounds of goods.  Once the Parthian kings received their cut of the profits from the trade, they reinvested much of it into their state of the art army.
The Parthian Military
The Parthian military was the vanguard of the empire’s territorial expansion, defeating several rebellious tribes in central Asia and fighting the Romans for control of Armenia and Mesopotamia. Cavalry comprised the most important core of the Parthian army and it was the nobility who were the overwhelming majority of the horse-borne fighters. The nobles were the only members of society who could afford horses, which they fought on wearing mailed armor with a variety of different weapons. In exchange for their service to the king, the noble cavalrymen were given an increased amount of autonomy in their own lands.  According to a variety of different ancient sources, the bow was the preferred weapon of the Parthians, which they put to great use on foot and horseback. The third century AD Roman historian Cassius Dio wrote:
“The Parthians make no use of a shield, but their forces consist of mounted archers and pikemen, mostly in full armour. Their infantry is small, made up of the weaker men; but even these are all archers. They practise from boyhood, and the climate and the land combine to aid both horsemanship and archery. The land, being for the most part level, is excellent for raising horses and very suitable for riding about on horse-back; at any rate, even in war they lead about whole droves of horses, so that they can use different ones at different times, can ride up suddenly from a distance and also retire to a distance speedily.” 
The Parthian tactic to attack and then retreat quickly that Dio mentions became known as the “Parthian Shot.” More specifically, the Parthian Shot involved the horsemen going into a tactical or even feigned retreat, enticing the enemy to give chase at a full speed. The Parthian horsemen then turned and shot arrows at their enemy as they retreated. Besides references made by Roman historians to the tactic, it was depicted in art as early as the late Hellenistic period. The earliest portrayals of the tactic are in a non-Parthian context, dated to eight and seventh century BC Assyrian and Phoenician art.  Numerous pottery, statues, and figurines, excavated in Mesopotamia and central Asia show horsemen, dressed in Parthian garb, riding while turning their heads and bodies to shoot at their enemies behind them.  The Parthians employed this tactic for their entire history because it was so effective; even when it did not prove to be decisive it still caused confusion among the enemy ranks.
The Parthians built one of the greatest and most powerful, although often overlooked, empires of the ancient world. For nearly 500 years, the Parthians ruled a large swath of land that stretched from Bactria to Mesopotamia and ruled over millions of different peoples. The Parthians were able to do this due to an excellent economy and military. Because Parthia was located strategically on the middle of the Silk Road, the Parthians were able to extract princely fees from merchants who wished to move goods between the West and East. After profiting from the Silk Road trade, the Parthian kings were able to fund a highly effective and unique army that relied on noble horsemen. Those horsemen expanded the Parthian Empire’s borders and successfully defended it from incursions by the Romans for many years before internal problems and a new Persian dynasty, the Sassanians, led to the collapse of the long-lived Parthian Dynasty.
- Colledge, Malcon A. R. The Parthians. (London: Thames and Hudson, 1967), p. 25
- Colledge, p. 68
- Brosius, Maria. The Persians: An Introduction. (London: Routledge, 2010), p. 81
- Boyce, Mary. Zoroastrians: Their Religious Beliefs and Practices. (London: Routledge, 2001), p. 95
- Brosius, p. 103
- Brosius, p. 103
- Colledge, p. 67
- Brosius, p. 110
- Brosius, pgs. 83-90
- Thorley, J. “The Silk Trade between China and the Roman Empire at Its Heights.” Greece and Rome 18 (1990) pgs. 73-77
- Brosius, p. 123
- Brosius, p. 116
- Cassius Dio. Roman History. Translated by Earnest Cary. (Cambridge, Massachusetts: Harvard University Press, 1954), XL, 15, 3-4
- Rostovtzeff, M. “The Parthian Shot.” American Journal of Archaeology 47 (1943) p. 180
- Rostovtzeff, p. 177