The ongoing hybrid warfare between Russia and Ukraine shows nationalistically charged rivalry. What causes such rivalry? The literature says that nationalism is a major cause of conflict, but it is under-theorized how nationalism provokes rivalry, i.e., a longer and more competitive process of hostility than conflict. To fill this gap, I synthesize social identity theory with interstate rivalry theory, arguing that nationalistic rivalries emerge if one state’s desire to maintain its national autonomy, unity, and identity is incompatible with another’s. Such incompatibility occurs if (1) both states are more ethnically heterogeneous, (2) both states experience more unstable domestic politics, or (3) the ethnonational and state borders are incongruent. Building on original data of nationalistic rivalries from 1946-2001, quantitative analysis supports my arguments. A policy implication for managing nationalist conflict is that while ethnic demography is difficult to change, addressing political instability in countries is feasible to prevent nationalistic rivalry.
