The formation of large polities has throughout history often involved the creation of new ethnic identities. Successful state-building is supposed to supplant parochial ethnic affiliations with a broader national identity. Violent internal conflict obviously plays a role in this process. When warfare unfolds over sectarian lines, it has the potential to strengthen in-group identification. As ethnic differences are emphasised by conflict and mistrust between groups soars, the appeal of a common national identity clearly fades away. Employing a novel dataset matching warring parties to ethnolinguistic groups in Sub-Saharan Africa, this paper presents empirical evidence on the ethnonationalist consequences of conflict and on their heavy drain on the state-building process.