Before activating Zhang Qian, think about which of your cities is likely to engage in the most foreign trade. In most cases, this is going to be your Capital or your most populous city. Civilizations with perks to foreign trade, such as Egypt, Mali, or Poland with its Sukiennice, should try to recruit Zhang Qian for further boosts.
Around 138 BC, Zhang Qian rode through the tall stone gates of Chang’an, capital of the Han dynasty, headed for the dry and unknown lands to the west of the empire. An officer of the imperial guard, he was at the head a contingent of 100 volunteers. On the northwest border of the Han were the barbarian Xiung-nu (otherwise known in the West as the Huns), a near constant threat. The Han needed allies to curtail the threat, and had heard rumors of the Yueh-chih beyond the Xiung-nu. Unfortunately, Zhang and his delegation were captured by the Huns, and spent the next decade as hostages.
Managing to extricate themselves from this unpleasantness, Zhang and some of his people crossed the Gobi and passed over the snow-capped Pamirs. They reached the Yueh-chih, who however had no interest in an alliance since the Xiung-nu didn’t bother them (much). So Zhang continued to wander, as far as northern India, and found a surprising number of peaceful and prosperous lands civilized people. The wealth and goods of these lands, not only their jade but agricultural products such as grapes and wine (unknown in China) and powerful horses, excited him. Moreover, these kingdoms proved quite interested in the Chinese trade goods, particularly the silk.
After further adventures, Zhang, his wife and son, and one other companion managed to return to Chang’an, 13 years after setting out. Although he had failed in his mission, Zhang's tales of the western lands and the potential for trade stirred the mercenary hearts of the Chinese. In short order the Han emperors sent forces to drive the barbarians away and open a route to the west, establishing garrisons to protect what would come to be known as the Silk Road, the richest trade route of history.