Simultaneously with the rise of the Qin and Han empires, some of the nomadic peoples of Central Asia, known as the Xiongnu, succeeded in achieving a measure of unity under a single leader. As a result, while the Chinese were consolidating their government, the lands lying to the north of the empire—and the northern provinces themselves—became subject to incursion by Xiongnu horsemen. One of the achievements of the Qin dynasty had been the unification of the several lines of defense into a single system of fortification, the Great Wall. By keeping that wall, or line of earthworks, manned, the Qin dynasty had been free of invasion. With the fall of Qin and China’s subsequent weakness, the wall fell into a state of disrepair and lacked a garrison. Until about 135 bce, Han governments were obliged to seek peaceful relations with the Xiongnu at the price of gold, silk, and even the hand of a Chinese princess. However, as Wudi’s governments began establishing strong policies, China took the offensive in an attempt to throw back the Xiongnu to Central Asia and to free the northern provinces from the threat of invasion and violence. By 119 bce, campaigns fought to the north of Chinese territory had attained this objective, and after a short interval it was possible to send Han armies to advance in the northeast (present-day North Korea), the south (present-day Vietnam), and the southwest. As a result of the campaigns fought from 135 bce onward, 18 additional commanderies were founded, and organs of Han provincial government were installed as outposts among peoples who were unassimilated to a Chinese way of life.

Chinese government was by no means universally accepted in those outlying regions. But despite large losses and expenditures incurred in fighting the Xiongnu, the Chinese were able to mount expeditions into Central Asia from about 112 bce. The defensive walls were repaired and remanned, and by about 100 they were extended to the northwest as far as Dunhuang. Chinese travelers, whether diplomats or merchants, were thus protected as far as the Takla Makan Desert. It was at about that time that trade routes skirting the desert were established and came to be known collectively as the Silk Road.

The success of Chinese arms in those remote areas was short-lived. Long lines of communication made it impossible to set up garrisons or colonies in the forbidding country to the west of Dunhuang. Diplomatic moves were made to implant Chinese prestige more firmly among the communities that were situated around the Takla Makan Desert and that controlled the oases; it was necessary for the Chinese to win those peoples’ support, thus denying it to the Xiongnu. In a few cases the Chinese resorted to violence or plots to remove a leader and to replace him with a candidate known to favor the Han cause. More commonly, one of the alien leaders was married to a Chinese princess, with the intention that he should in time be succeeded by an heir who was half-Chinese. These endeavors and the military ventures met with partial success. While the Chinese position in Central Asia was subject to question, relations with the Xiongnu leaders varied. The visit of a Xiongnu leader to Chang’an in 51 bce was hailed as a mark of Chinese success, but the ensuing decades were not free from fighting. Chinese prestige declined toward the end of the Xi Han and recovered only during the reigns of Mingdi and Zhangdi, when the Han government was once more strong enough to take the field. Ban Chao’s campaigns in Central Asia (from 94 ce) reestablished the Chinese position, but again the full strength of Chinese prestige lasted for only a few decades. During the Dong Han, China suffered invasion from the northeast as well as from the north. The settlement of Xiongnu peoples south of the wall was a disruptive factor in the 2nd and 3rd centuries, to the detriment of imperial unity.

The Han expansion into Central Asia has been represented by the Chinese as a defensive measure designed to weaken the Xiongnu and to free China from invasion. Allowance must also be made for commercial motives. Some of Wudi’s statesmen were well aware of the advantages of exporting China’s surplus products in return for animals and animal products from Central Asia, and there is evidence that Chinese silk was exported at this time. No attempt can be made to estimate the volume of trade, and, as the transactions were conducted through Parthian middlemen, no direct contact was made by this means between Han China and the world of Rome and the Mediterranean. China’s export trade was sponsored by the government and not entrusted to private merchants.

The Great Wall formed a boundary separating the Chinese provinces from the outside world. Traffic was controlled at points of access, not only to check incoming travelers to China but also to prevent the escape of criminals or deserters. At the same time, a ban was imposed on the export of certain goods such as iron manufactures and weapons of war. The wall also formed a protected causeway for travelers to the west. Watch stations were erected in sight of each other to signal the approach of the enemy, and the garrison troops were highly trained and disciplined. Meticulous records were kept to show how government stores were expended and rations issued; routine signals were relayed along the line and daily patrols were sent out to reconnoiter.

As a result of the campaigns and diplomatic activity, China’s immediate contacts with other peoples grew more brisk. Many of the Xiongnu and other neighboring leaders who had surrendered to Han arms were given nobilities and settled in the interior of the empire. Zhang Qian was a pioneer who had set out about 130 bce to explore the routes into Central Asia and northern China, and, as a result of his report and observations, Han advances were concentrated in the northwest. In 97 ce Chinese envoys were frustrated in an attempt to visit the western part of the world, but a mission from Rome reached China by ship in 166. The first record of official visitors arriving at the Han court from Japan is for the year 57 ce.