Why do the borders of nation-states often fail to match the boundaries of national groups?

While nationalism aspires to a perfect congruence between state borders and the boundaries of a national community, this ideal is rarely achieved in practice. The borders of nation-states often fail to match the boundaries of national groups for several fundamental reasons:1

• Demographic Complexity: In reality, state borders and national boundaries only partially overlap. Most states contain diverse populations where not all residents belong to the core national group, and conversely, members of a specific nation often reside across multiple different states.1
• Historical Formation: Many states were not formed by organic national movements but through the transformation of existing polities, such as the retreat of colonial empires or the transition from monarchies to parliamentary systems. This often resulted in borders drawn by external powers or historical accidents rather than the distribution of ethnic or cultural groups.12
• Conflicting Principles: The nation-state model attempts to fuse state sovereignty with national sovereignty. Because these principles prioritize different forms of legitimacy, they frequently clash with the reality of multi-ethnic societies, leading to ongoing challenges regarding citizenship, minority rights, and secessionist aspirations.1345