Bactria was a satrapy of the Achaemenid Empire. The first mention of Bactria comes in 520 BCE at the Behistun inscription. Bactria was a special satrapy in that it was ruled by a crown prince or an intended heir. The capital of Bactria was Bactra, and the region also sometimes included Sogdia. During the reign of Darius the Great, the Bactrians and the Aeglians were placed in one tax district, which was supposed to pay 360 talents every year.