This chapter explores ethnic militaries in the Achaemenid Empire. It examines two groups of potential military diasporas—ethnically defined (non-Persian) troops on the one hand and Persian contingents on the other—and relates the findings to the world view of the Achaemenid kings. It concludes that because of the political framework and self-understanding of the Great King as the ruler of a universal empire, there could be no form of military diaspora, neither inside nor outside the empire strictu sensu. A different matter is, of course, the individual self-perception of the soldiers, which cannot be addressed due to a lack of sources. The chapter thus highlights scholars’ difficulties in moving beyond a modern focus of diasporas within the context of nation states. The case of both Persian and non-Persian military groups in the Achaemenid empire seems to be paradigmatic for the use of diasporic militaries in the management of empires.