This essay examines the environmental history of the Afghan frontier city of Balkh and its decline in the Central Eurasian caravan trade in the early nineteenth century. Situated on the frontier between the steppe and the sown, with the Hindu Kush to the south and the Oxus River to the north, the city of Balkh was an oasis at the intersection of caravan routes leading to India in the south, to Iran in the west, and to China in the east. Though romanticized by travelers as a fallen and forgotten city built by Alexander and destroyed by the Mongols, into the nineteenth century Balkh continued to serve as a market for the Central Eurasian horse trade. But as the city's irrigation canals deteriorated and its environs became a swamp, it was visited by repeated outbreaks of cholera and malaria, until the city was abandoned for the nearby shrine town of Mazar-e Sharif in the mid-nineteenth century. The decline of Balkh was chronicled in the writings of nineteenth-century explorers, travelers, and their surveying missions. Based on a reading of Persian and European travel narratives, this essay chronicles the decline of Balkh and its transformation from a center to a periphery of the Central Eurasian world.