Xiongnu
Sinologist Edwin Pulleyblank argued that the Xiongnu were part of a Xirong group called Yiqu, who had lived in Shaanbei and had been influenced by China for centuries, before they were driven out by the Qin dynasty. Qin's campaign against the Xiongnu expanded Qin's territory at the expense of the Xiongnu. In 215 BC, Qin Shi Huang sent General Meng Tian to conquer the Xiongnu and drive them from the Ordos Loop, which he did later that year. After the catastrophic defeat at the hands of Meng Tian, the Xiongnu leader Touman was forced to flee far into the Mongolian Plateau. The Qin empire became a threat to the Xiongnu, which ultimately led to the reorganization of the many tribes into a confederacy.In 209 BC, three years before the founding of Han China, the Xiongnu were brought together in a powerful confederation under a new chanyu, Modu Chanyu. This new political unity transformed them into a more formidable state by enabling formation of larger armies and the ability to exercise better strategic coordination. The Xiongnu adopted many of the Chinese agriculture techniques such as slave labor for heavy labor, wore silk like the Chinese, and lived in Chinese-style homes. The reason for creating the confederation remains unclear. Suggestions include the need for a stronger state to deal with the Qin unification of China that resulted in a loss of the Ordos region at the hands of Meng Tian or the political crisis that overtook the Xiongnu in 215 BC when Qin armies evicted them from their pastures on the Yellow River.
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.