This chapter proposes a sociological reconstruction of the emergence of citizenship as a source of legitimacy for political institutions, and it focuses on examining the historical processes that first gave rise to this concept. It explains how citizenship has its origins in the transformation of feudal law, a process that culminated in patterns of military organization that characterized the rise of the early modern state in Europe. On this basis, it describes how the growth of constitutional democracy was integrally marked by the militarization of society and explains that military pressures have remained palpable in constitutional constructions of citizenship. In particular, it argues that, through the early growth of democracy, national citizenship practices were closely linked to global conflicts, and they tended to replicate such conflicts in national contexts. It concludes by showing how more recent processes of constitutional norm formation, based largely in international human rights law, have acted to soften the military dimensions of citizenship.