Han dynasty
To the north of China proper, the nomadic Xiongnu chieftain Modu Chanyu (r. 209–174 BC) conquered various tribes inhabiting the eastern portion of the Eurasian Steppe. By the end of his reign, he controlled Manchuria, Mongolia, and the Tarim Basin, subjugating over twenty states east of Samarkand. Emperor Gaozu was troubled about the abundant Han-manufactured iron weapons traded to the Xiongnu along the northern borders, and he established a trade embargo against the group. In retaliation, the Xiongnu invaded what is now Shanxi province, where they defeated the Han forces at Baideng in 200 BC. After negotiations, the "heqin" agreement in 198 BC nominally held the leaders of the Xiongnu and the Han as equal partners in a royal marriage alliance, but the Han were forced to send large amounts of tribute items such as silk clothes, food, and wine to the Xiongnu.
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.