During the mid to late first millennium , Central Asia underwent enormous political and cultural change. Through the military campaigns of the Achaemenid kings Cyrus II and Darius I, Central Asia came to form the empire's northeastern frontier by the late sixth century . 1 As with the rest of the Achaemenid Empire, this space was subdivided into dahyava (countries)-preexisting cultural units refigured as satrapal administrative units-from which taxes, military personnel, and laborers could be levied. Bactria was an especially resource-rich and significant country, whose satraps were often drawn from the Achaemenid royal family, but it was also unruly and prone to revolt. 2 Here, the Achaemenid administration further developed the irrigation systems that had existed since the Bronze Age, 3 and introduced the use of Aramaic as a chancellery language; the use of Elamite has been additionally attested in Arachosia. 4 Between 329 and 327 , Alexander the Great campaigned in Central Asia for two difficult years in the pursuit of Bessos (the satrap of Bactria who had murdered the king Darius III), all the while adapting himself to Persian modes of rule and largely preserving administrative structures he found in place. 5 One important cultural ramification of Alexander's conquests was the introduction of demobilized Greek and Macedonian soldiers into the population through the foundation of new settlements with a strong military component. 6 Garrisons were established within existing cities or as new outposts, existing cities were refounded, and new foundations were also established, such as Alexandria Eschate on the Jaxartes (mod. Syr