Through excessive mobilization of resources, imperial governments caused environmental damage and generated social tensions and economic difficulties by concentrating too much wealth in the hands of elites: deforestation, desertification, soil erosion, silted rivers. B. External problems resulted from the security issues along their frontiers, including the threat of invasion: between Han China and the Xiongnu, between the Gupta and the White Huns, between the Romans and their northern and eastern neighbors.
Emergence of Transregional Networks of Communication and Exchange I. Land and water resources became the basis for transregional trade, communication, and exchange networks in the Eastern Hemisphere. A. Many factors, including the climate and location of the routes, the typical trade goods, and the ethnicity of people involved, shaped the distinctive features of a variety of trade routes: Eurasian Silk Roads, Trans-Saharan caravan routes, Indian Ocean sea lanes, Mediterranean sea lanes II. New technologies facilitated long-distance communication and exchange. A. New technologies permitted the use of domesticated pack animals to transport goods across longer routes: yokes, saddles, stirrups, horses, oxen, llamas,