By the end of the first century B.C., there was a great expansion of international trade involving five contiguous powers: the Roman empire, the Parthian empire, the Kushan empire, the nomadic confederation of the Xiongnu, and the Han empire. Although travel was arduous and knowledge of geography imperfect, numerous contacts were forged as these empires expanded—spreading ideas, beliefs, and customs among heterogeneous peoples—and as valuable goods were moved over long distances through trade, exchange, gift giving, and the payment of tribute. Transport over land was accomplished using river craft and pack animals, notably the sturdy Bactrian camel. Travel by sea depended on the prevailing winds of the Indian Ocean, the monsoons, which blow from the southwest during the summer months and from the northeast in the fall.
A vast network of strategically located trading posts (emporia) enabled the exchange, distribution, and storage of goods. Isodorus of Charax, a Parthian Greek writing around 1 A.D., described various posts and routes in a book entitled Parthian Stations. From the Greco-Roman metropolis of Antioch, routes crossed the Syrian Desert via Palmyra to Ctesiphon (the Parthian capital) and Seleucia on the Tigris River. From there the road led east across the Zagros Mountains to the cities of Ecbatana and Merv, where one branch turned north via Bukhara and Ferghana into Mongolia and the other led into Bactria. The port of Spasinu Charax on the Persian Gulf was a great center of seaborne trade. Goods unloaded there were sent along a network of routes throughout the Parthian empire—up the Tigris to Ctesiphon; up the Euphrates to Dura-Europos; and on through the caravan cities of the Arabian and Syrian Desert. Many of these overland routes ended at ports on the eastern Mediterranean, from which merchandise was distributed to cities throughout the Roman empire.
Other routes through the Arabian desert may have ended at the Nabataean city of Petra, where new caravans traveled on to Gaza and other ports on the Mediterranean, or north to Damascus or east to Parthia. A network of sea routes linked the incense ports of South Arabia and Somalia with ports in the Persian Gulf and India in the east, and also with ports on the Red Sea, from which merchandise was transported overland to the Nile and then to Alexandria.