Excerpts from Books and Wikipedia
"The Han court dispatched Zhang Qian, a military officer who was familiar with the Xiongnu, to the Western Regions in 138 BC with a group of ninety-nine members to make contact and build an alliance with the Yuezhi [in modern Tajikistan] against the Xiongnu. . . . However to get to the territory of the Yuezhi he was forced to pass through land controlled by the Xiongnu who captured him (…) and enslaved him for ten years. During this time he married a Xiongnu wife, who bore him a son, and gained the trust of the Xiongnu leader. . . . 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 . . . Of the original mission of just over a hundred men, only Zhang Qian and Ganfu managed to return to China. Zhang Qian returned in 125 BC with detailed news for the Emperor, showing that sophisticated civilizations existed to the West, with which China could advantageously develop relations." [Wikipedia]