Along its northern and western borders, the Han a could not establish a tributary system. b promoted the establishment of garrison cities and oases as military and farming settlements that became part of the Silk Road. c used the western deserts to defend themselves militarily but at the cost that trade did not develop in this region. d permitted regional lords to dominate and control these barren lands. e faced an enforced limit to its empire because of the Xiongnu tribesmen, whom the Han could not subdue.