How did Persia Control the Silk Road?
By control of the central section, by tariffs at the principal cities and borders, by a network of caravanserais, by protection of merchants and caravans, by treaties with the powers on either side, and — at one famous moment in 97 CE — by deliberately misleading a Chinese envoy. The Persian empires, in particular the Parthian and Sasanian dynasties, controlled the Silk Road for almost the entirety of its history, and the methods of control were varied. The cluster on the Silk Road and the Persian Empire gives the broader context; the long-tail here is the question of how the control worked.
The geography of control
The Persian-controlled Silk Road ran through the heart of the Iranian plateau, from the oasis cities of Khorasan in the east to the Mesopotamian cities of Baghdad and Ctesiphon in the west. The principal routes were the northern route, which passed through Merv, Nishapur, Rayy, Hamadan, and Baghdad, and the southern route, which passed through Herat, Yazd, Isfahan, and Susa. The two routes converged in Mesopotamia, where the goods were shipped to the Mediterranean ports of Antioch, Tyre, and Alexandria.
The Persian state was uniquely positioned to control this trade. It held the central section of the Silk Road, the only viable east-west route in many places, and it could, in principle, close the route to traffic at will. The mere threat of closure was a powerful lever in negotiations with both eastern and western powers. The Chinese, Indian, and Roman empires, in contrast, could only trade through the Persian-controlled routes, and they were therefore at a structural disadvantage in the commercial relationship.
Tariffs and customs duties
The principal method of Persian control of the Silk Road was the imposition of tariffs and customs duties. The Parthian and Sasanian states both maintained an elaborate system of customs posts, typically located at the major cities and at the borders of the empire, where merchants were required to declare their goods and pay a duty. The typical rate, by the Sasanian period, was about 10 per cent of the value of the goods, although the actual rate varied depending on the type of goods, the origin and destination of the merchant, and the political relationship between Persia and the merchant’s home state.
The Sasanian customs system was particularly elaborate. The mavaset (or customs post) was established at every major city and at the principal borders. The customs officials, the mustaufi, kept detailed records of the goods, the merchants, and the duties paid, and they were accountable to the central treasury. The customs revenue was a major source of imperial income, and the Sasanian kings invested heavily in the maintenance of the customs system. The Chinese sources for the early Tang period and the Arab geographers of the ninth and tenth centuries are the main witnesses to the system; the earlier Sasanian administrative documents are largely lost, and what we have is the Islamic and post-Islamic reconstruction of the older system.
Caravanserais and the infrastructure of the trade
The Persian state maintained a vast network of caravanserais, or robats, along the routes of the Silk Road. The caravanserai was a fortified inn, typically built of stone or brick, with a large central courtyard surrounded by vaulted cells for the merchants and stables for the animals. The caravanserais were built at intervals of about a day’s march along the major routes, and they were typically maintained by the local governor or by a pious foundation. The merchants paid a small fee for the use of the caravanserai, and they were protected from bandits, robbers, and the elements.
The Sasanian caravanserais were among the most elaborate in the ancient world. The caravanserai of Bishapur, the Robat-i Sharaf on the road from Nishapur to Merv (built in the 12th century but reflecting the Sasanian tradition), and many other surviving examples attest to the high quality of Sasanian construction. The caravanserais were typically built around a central courtyard, with a monumental iwan on one side, vaulted cells on the other sides, and corner towers. The plan was derived from the Sasanian palace, and the caravanserai functioned, in practice, as a small palace for the use of the travelling merchant.
The Persian state also maintained the roads themselves, with regular repairs, the construction of bridges, and the provision of water supplies. The roads were patrolled by royal garrisons, and the merchants were protected by the king’s peace. The combination of the caravanserais, the roads, the garrisons, and the royal protection made the Persian-controlled Silk Road one of the safest and most efficient trade routes in the pre-modern world.
Protection of merchants and caravans
The protection of merchants and caravans was a central element of Persian control of the Silk Road. The Persian state maintained a system of rahdars, or road-guards, who patrolled the major routes and protected the caravans from bandits. The rahdars were typically drawn from the local population, often from the tribal or nomadic peoples of the Iranian plateau, and they were paid by the merchants for their services. The system was not always efficient, and banditry remained a serious problem, particularly in the eastern Iranian plateau and in the mountains of Khorasan. But the relative safety of the Persian-controlled routes, compared with the alternatives, was a major attraction for the merchants of the east and the west.
The Persian state also maintained a system of pashtiban, or military escorts, for the larger caravans. The pashtiban were typically drawn from the regular army, and they accompanied the caravans through the most dangerous sections of the route. The system was expensive, and it was typically reserved for the larger and more valuable caravans, but it was an important element of the overall protection system.
Diplomatic treaties and the Sasanian peace
The Persian control of the Silk Road was underpinned by a series of diplomatic treaties with neighbouring states. The most famous of these is the Peace of 562 between the Sasanian king Khosrow I and the Byzantine emperor Justin II (not Justinian I, who died in 565; the 562 treaty was the result of negotiations that had been ongoing under Justinian I, but the final agreement was between Khosrow I and his successor Justin II). The treaty included detailed provisions for the regulation of the silk trade, with Sasanian silk to be sold to Byzantium at fixed prices and the proceeds to be divided between the two empires, and it also included provisions for the extradition of political refugees, the regulation of border trade, and the conduct of diplomacy.
The Sasanian state also maintained diplomatic relations with the Sogdian, Bactrian, and Tokharian principalities of Central Asia, with the Indian powers of the subcontinent, and with the Chinese Tang dynasty. The treaties typically included provisions for the protection of merchants, the regulation of tariffs, and the exchange of diplomatic gifts. The Sasanian kings used the treaties to maintain the flow of goods through their territory and to keep the prices of eastern goods high in the Mediterranean.
Information management and the Parthian strategy
The Persian state also used information management as a tool of control. The famous story of Gan Ying, the Chinese envoy sent to seek an alliance with Rome in 97 CE, is a striking example. According to the Hou Hanshu, Gan Ying reached the Parthian court and was there dissuaded from continuing westward by the Parthian king, who convinced him that the Parthian Ocean (the Mediterranean) was unnavigable and that direct contact with Rome was impossible. Gan Ying returned to China, and the Han court abandoned its plan of direct contact with Rome. The Parthians, in other words, used misinformation to keep the Chinese and the Romans apart, and to keep the silk trade in their own hands.
The Sasanians continued this policy. The famous letter of Khosrow I to the Byzantine emperor, in which the Sasanian king boasted of his control of the silk trade and threatened to disrupt the supply of silk to Byzantium, is a similar exercise in information management. The Sasanians, like the Parthians, were masters of the diplomatic game, and they used the Silk Road as a major element of their foreign policy.
The Sogdian merchants and the Sasanian administration
The Sogdian merchants, an Iranian people of Central Asia, were the principal commercial agents of the Sasanian Silk Road. The Sogdians, who spoke a Middle Iranian language and used a Sogdian script, established communities at every major node of the trade, from Samarkand and Bukhara in the east to Chang’an in China and the bazaars of Baghdad and Damascus in the west. The Sasanian state relied on the Sogdians for the day-to-day administration of the Silk Road, and the Sogdian merchants carried not only goods but also religions, particularly Manichaeism and Buddhism, between the major civilisations of Eurasia. Étienne de la Vaissière’s Sogdian Traders (2002, English 2005) is the standard recent work on the Sogdian commercial network.