There does not appear to be much evidence for long distance trade on the Silk Road prior to the third century BC. It is possible that some did exist and the evidence has not persisted, but for many reasons, it is more likely to have been the exception. In examining the effects of polities on trade, the first duty will be to credit the growth of various empires with making such trade possible in the first place.
The stability of the Han empire in unified China, the Kushan Empire of Central Asia, the Parthian Empire to its west and the waxing Roman Empire completed a chain of trade zones which for the first time sent Silk Road trade into high gear. Several factors must have been at work in this. The formation of such empires must have for the first time led men to think in terms of large polities and their relations and travel to the outside. At the same time, travel within and through the areas of the newly-constituted empires could finally be accomplished. Civil and military authority could be expected to enforce peace and some semblance of fairness in dealings. Roads and way stations had a better chance being maintained. There was less chance of being captured by raiding nomads who typically coerced merchants into tribute or worse, held them for ransom. Tribute had been a problem too in the pre-empire days when each tiny kingdom and satrapy had demanded a large fee for the privilege of trade, or even traversal. Finally, the standardization of weights and measures, even if different in each empire, must have been incredibly useful in helping traders to receive a fair price from station to station.
For these reasons, the best days for the Silk Road are certainly to be found in four distinct periods: (1) the one already mentioned which lasted until the Han disintegration in about the third century, (2) the period under Tang unification, with Islamic polities and the Byzantine Empire providing the other links, roughly the seventh to the early tenth centuries, (3) the "Pax Mongolica" after 1250 when the Mongols had conquered most of the route up to the Byzantine Empire, to 1368 and (4) the final, lesser flowering of the short period of the Timurid Empire in the fourteenth century despite Ming resistance in China. In most cases, all the empires named were quite conscious of the benefits of trade and actively encouraged and supported it, although not always in the wisest fashion.
Trade brought wealth, not just to the few hundred or perhaps low thousands of individuals who partook in it, but to the entire region in which they operated. Traders passing through oasis towns left some fraction of their wealth there in exchange for stabling for their animals, shelter for themselves, food, drink and entertainment after a long and dusty day on the road. In towns like Dunhuang which was no doubt a terminus for traders of Chinese origin, they endowed vast sculptures and artworks such as those at the Mogao Caves and this must have been true in many places along the road. Besides being difficult, long distance trade was dangerous work and many a trader must have inwardly promised a large donation to the local deity if only he could survive the next day's journey through nomadic territory or through a dangerous Pamir mountain pass. There were also taxes and border tolls to pay even in the imperial period and before one could start trading in a town for the first time, a visit to the local potentate to pay tribute was necessary. Finally, a long distance trader brought wealth in the form of rare items which might have much greater value than they did in the land of their origin, taking back with him items of similar value for his own country. In this way, the trader performed a true service for both lands in increasing the values of their economies. This was the the basis of the flowering of civilization, of advances in the arts, the sciences and the humanities, of which the Mogao Caves are but one example. However, as we shall see, all too often the polities involved could not leave well enough alone and attempted to have their cake as well as eat it.
One such group would seem to have purer motives in the sense that they seemed excluded from trade and sought to remedy it. These were the Xiong Nu nomads. In the first two centuries AD they provoked constant turmoil in northwestern China. In the early days of the road, they simply made trade more dangerous since their presence north of the Tien Shan tended to discourage journeys in that area, forcing use of the more perilous desert travel skirting the Taklamakan. But in 39 they occupied Shanxi, in 46 Yarkand, Lou-lan, Turfan and Kucha and in 74 even dared to attack Gansu. Chinese counterattack drove them back, but after 150, the western marches were simply written off. Actually, earlier in their history the Xiong Nu had been the beneficiaries of trade – they seem to have been providers of camels, the backbone of Serindian trade itself, to the traders, but seeing that they might take more, they not only overran the newly-grown up settlements and seized their wealth, they also attacked trade caravans and exacted tribute or worse. Since in the event they did not provide the civilizing effects of empire, the end result was a net loss for all concerned including the Xiong Nu as silk road trade virtually ceased for a few centuries.
When the land route dried up, some trade found its way over the sea. This trade is mostly beyond the scope of this paper, except to point out that it was to the partial benefit of the Persian empire as even though the volume of trade had probably decreased, the fact that ships could not avoid stopping at ports for fresh supplies of food and water and most of the ports were in Persian control meant that for the first time Persia could usually maintain total control over long distance trade. In addition, more profits could be realized by Persians and for greater Persia as well since the sea route meant fewer middlemen whereas Rome and the Byzantine Empire would continue to pay the same prices.
The Parthians, predecessors of Sassanid Persia, had been, like the Kushans, solicitous of trade. The rulers even caused to be set up along the route caravanserais at various intervals, but they did tend to reserve these same facilities for traders of their own country, preferring foreigners to remain at the border. In the cases of both empires, this was probably to their overall detriment since it meant that trade was less than it might otherwise have been and that hardy traders would seek their way outside their borders. When the Parthians were weak, they took a southern detour and arrived at Petra in the West, bypassing the Parthians altogether. Similiarly, when the Persians refused to deal with the Turks or the Sogdians who belonged to their kingdoms, the traders found a northern way which took them over the Caucasus and thence into Anatolia.
Imperial interference in trade was also to be found in the West. The same Justinian who is considered so wise for his books of law and so great for his reconquest of barbarian-occupied Roman lands, tried not only to gain all the wealth from the silk trade by monopolizing it, but tried to limit the flow of the gold used to pay for it. As later practitioners of price controls have discovered, it is a difficult trick to make work and the results were that all the silk and silk-oriented businesses in the Byzantine Empire promptly went out of business or moved over the border into Persia, which could not have been a favorable development for the treasury, and, the Persians, faced with an import quotas, simply did as did the Japanese automakers in our own era, they raised their prices to make up the difference.
But such apparently self-immolating practices were not unique. In Canton in 878, a coup left a certain Hung Ch'ao holding the reins of power. His first move was to chop down all the mulberry trees and expel foreign traders. As a result, the pendulum swung and the silk trade that had gone overseas now returned to the land routes. This development had been made possible because the Sui and then the Tang Dynasty (618-907) had managed to reunite China and in particular extend power back out to the northwest and even to Tibet. The states of Central Asia were in a tributary relationship and China welcomed foreigners to its markets. In 731, the Tang had officially recognized money which was must have been a boon to traders as they now could receive payment in a universally acceptable medium. A new tax system which permitted payment in money assisted them in similar ways. Silk road trade was in its golden age.
Again, greed was not to permit a long duration. Eventually the Tang must have felt it was not getting sufficient worth from trade and started to institute monopolies of the type we have already seen, this time on salt, iron, and even wine. Sales of some items to foreigners began to be prohibited. Arabs started having to surrender fully one-third of their cargoes in China, a steep tax indeed. Declining wealth and governmental control as well as an increasing inwardness in China might well be studied in this context. At the same time, control of the mountain fastnesses of Tibet was not easily maintained and having had a taste of trade, its raiders struck and threatened it. In 763 they overran China and in 800 exerted virtual control over the northwest area. Their power continued well into the 1100's. In a sense, it was the story of the Xiong Nu all over again. Trader expenses mounted as they were forced to hire out troops of archers to escort their expeditions. And some switched back to use of sea routes.
These sea routes were still monopolized by the Persians, but then the arrival of the Arabs restricted trade even more. More religious in their statecraft than the Persians had been, the Arab states looked askance at any non-Islamic traders. Chinese and Western traders alike were excluded from the new states. Alone among all groups, the Jews, being neither Christian nor Moslem, could take advantage of this situation. Generally tolerated by the usually intolerant West (Charlemagne was an explicit patron of Jewish groups), they were allowed to trade in Islamic lands with only the hindrance of a special tax enacted by the Abassids in the eighth and ninth centuries. Jewish traders became so important that they came to exert a controlling influence in the government of Khazaria, perhaps the only case of traders having a direct influence on a government rather than the reverse.
At the same time however, Silk Road trade had been fundamentally changed because of the exclusion of the Western Christendom. It may be that this effect was fundamental to the "European Dark Ages" and perhaps should be further investigated. In the meantime, however, states such as Morocco and Spain which had not much participated in Silk Road trade now became involved as they represented full-fledged components of the Islamic world. This reality was in turn disturbed by the Turks who in the ninth century entered Central Asia, initially behaving much as did the Xiong Nu. Their innovation, however, was to adopt the sedentary life and rule as established kingdoms, no doubt advised by an Islamic bureaucracy already present. Thus after the Seljuq invasion of the eleventh century, trade was able to resume much as normal.
The Mongols have a terrible reputation as the horsemen of the devil himself, but perhaps this needs revision. Their invasions of the early thirteenth century gained them Central Asia in 1230 and a reunited China by 1260. This put the entire Silk Road in Mongolian hands right up to the Syrian coast. And even if Syria was closed to them, Trebizond was anyway available. As attested by the writings of Marco Polo, foreigners and their ideas (even religious ones) and products were welcomed by the khans leading to a rejuvenation of the route and also to the Eurasian Steppe Route as a reflection of the northern orientation of the Mongols. The Pax Mongolica even supported a postal system with all the way stations and supports that this implies, certainly of no small assistance to traders. The mini-Renaissance in Europe, sometimes termed a renascence, during this time, 1250-1368, might profitably be studied in this context as the Mongols, despite their depredations in Europe, proved to be an effective ally with the West against the Moslem states.
The Mongolian empire was too unwieldy to survive and there was some apparent decline in silk road trade with its passing. There was a final flowering however under the aegis of Timur who demonstrated that he understood quite well the benefits of long distance trade, in fact conquering much of the traditional route. His empire however was even shorter-lived and anyway, the silk road was at last in its decline as Henry the Navigator and a small school in the southwestern corner of Europe prepared to discover a sea route from Europe to China and change the world forever.