Zhang Qian was sent by the Han Chinese emperor to enlist the help of a distant nation in war against a common enemy. Zhang was unable to form an alliance, instead, he formed the relationships and pathways that led directly to the silk road, connecting China to the west and, eventually, to Rome.
Sima Qian, Shiji (Records of the Grand Historian), 1st Century BC
Friedrich Hirth, "The Story of Chang K'ien, China's Pioneer in Western Asia: Text and Translation of Chapter 123 of Ssi-ma Ts'ien's Shi-Ki," Journal of the American Oriental Society, 1917
Cambridge History of China, Volume 1, Chapters 2 and 6, 1986
Xinru Liu, Journal of World History, Migration and Settlement of the Yuezhi-Kushan: Interaction and Interdependence of Nomadic and Sedentary Societies, 2001
Walter Perceval Yetts, "Chinese Contact with Luristan Bronzes", The Burlington Magazine for Connoisseurs, August 1934