This article retraces the parallel and contrasting developments of state formation and of citizenship in Mauritania, recasting the reflection on postcolonial and anthropological debates on citizenship and state and civil society. In this perspective, cultural, ethnic and even “racial” differences – such as the Arabs/Africans or White/Black peoples dichotomies – have alternatively been considered as a social resource for consolidating a postcolonial nation or a threat to social harmony and to political development. The article deconstructs both of these positions in order to show their common features in their tendency to reduce state and civil society relationships to a matter of “horizontal” interactions between social groups. The hypothesis is that these visions have historically played a depoliticizing role, hiding the “vertical” dimension of relationships between hegemonic governing elites and social groups that are economically and socially fragmented, hierarchized, and even discriminated against. The article proceeds in three steps. First, it shows the way in which issues of identity are highly sensitive in contemporary Mauritania, relying particularly on a recent case of ethnic discrimination during a census campaign. It then retraces the evolution of political and intellectual debates on identities in Mauritanian society, putting them in perspective with the evolution of political power or of the political interests and views of social and political actors. Finally, it relies on historical and ethnographic records about a particular social group (a pastoral Fulani lineage), which does not fit into usual ethnic categories and dichotomies, and by that ultimately shows the political value of discourses on identity.