This dissertation examines the divergent trajectories of ethnic and national politics in the Tamil speaking regions of India and Sri Lanka. Despite comparable historical experiences and conditions, the south Indian Tamil speaking areas were peaceably accommodated within a pan-Indian framework whilst Sri Lankan politics was marked by escalating Tamil-Sinhala ethnic polarisation and violent conflict. The dissertation explains these contrasting outcomes by setting out a novel theoretical framework that draws on the work of Reinhart Koselleck and his analysis of the links between concepts and political conflict. It argues that in the era of popular sovereignty the nation and ethnicity have become central and unavoidable concepts of political order, but concepts that can be deliberately constructed through political activity in more or less inclusive ways. Setting out the conceptual connections between the nation, ethnicity and popular sovereignty, the dissertation shows how the conceptual tension between a unified national identity / interest and ethnic pluralism becomes a central and unavoidable locus of political contestation in the era of popular sovereignty. Tracing the politics of ethnicity and nationalism in India and Sri Lanka from the late nineteenth century to the late 1970's, the analysis shows that the accommodation of Tamil identity within Indian nationalist frameworks and the escalation of Tamil - Sinhala ethnic conflict in Sri Lanka cannot be linked to differences in ethnic demography, political system, historical experiences or the structure of economic incentives. It reveals instead that these divergent outcomes are best explained as effects of contingent and competitive processes of political organisation and mobilisation through which deliberately more or less ethnically inclusive national identities are asserted, established and then contested.