Nomadic empire
The Xiongnu were a confederation of nomadic tribes from Central Asia with a ruling class of unknown origin and other subjugated tribes. They lived on the Mongolian Plateau between the 3rd century BC and the 460s AD, their territories including modern day Mongolia, southern Siberia, western Manchuria, and the modern Chinese provinces of Inner Mongolia, Gansu, and Xinjiang. The Xiongnu was the first unified empire of nomadic peoples. Relations between early Chinese dynasties and the Xiongnu were complicated and included military conflict, exchanges of tribute and trade, and marriage treaties. When Emperor Qin Shihuang drove them away from the south of the Yellow River, he built the famous Great Wall to prevent the Xiongnu from coming back.
Zhang Qian
Zhang and Ganfu (as well as Zhang's Xiongnu wife and son) were eventually able to escape and, passing Lop Nor and following the northern edge of the Tarim Basin, around the Kunlun Mountains and through small fortified areas in the middle of oases in what is now Xinjiang until they made their way to Dayuan and eventually to the land of the Yuezhi. The Yuezhi were agricultural people who produced strong horses and many unknown crops including alfalfa for animal fodder. However, the Yuezhi were too settled to desire war against the Xiongnu. Zhang spent a year in Yuezhi and the adjacent Bactrian territory, documenting their cultures, lifestyles and economy, before beginning his return trip to China, this time following the southern edge of the Tarim Basin.
On his return trip he was again captured by the Xiongnu who again spared his life because they valued his sense of duty and composure in the face of death. Two years later the Xiongnu leader died and in the midst of chaos and infighting Zhang Qian escaped. Of the original mission of just over a hundred men, only Zhang Qian and Ganfu managed to return to China.