This chapter explores the relationship between ethnopolitics and nationalism, and more specifically how ethnic identity contributes to war and the amelioration of ethnic conflicts. It first considers the construction and politicization of ethnic identities — in other words, the construction of ethnic and ethnopolitical identities — before discussing the construction of a variety of nationalist identities in the developing world. It then examines the conflictual, competitive, and cooperative interactions of groups based on nationalist identities with one another and with states, along with states’ efforts to mould these interactions in ways that enhance the legitimacy of state-based nations and their support from various groups. The chapter shows that cooperative interactions tend to promote nation-building through multi-ethnic/multicultural nationalism.