Drawing on theories of nationalism and interstate rivalry, the thesis develops a newtheory of nationalistic rivalry, i.e., a specific type of rivalry where dispute-prone states perceive each other as threatening and competing enemies due to a nationalist issue. Building on original data of nationalistic-rivalry dyads from 1946-2001, the thesis examines the validity of nationalistic rivalry theory through the combination of large-N statistical analysis and small-N case studies. It finds that nationalistic rivalry significantly increases the probability of revisionist foreign policy, and thwarts the effect of well-known conflict-mitigating factors. The literature has found that nationalism is a major cause of conflict. However, it is little known whether nationalism in the context of interstate politics increases revisionist state behavior specifically. While revisionist states have often been nationalistic (e.g., Imperial Japan or Milosevic Serbia), nationalism is not always revisionistic (e.g., the UK at the time of the Falklands War). What conditions cause nationalism to be a force of revisionist behavior? The thesis answers this question by explaining how nationalist politics could lead the state specifically to revisionist behavior. The major findings of the thesis are: (1) an ethnically heterogeneous society, political instability, and incongruence between ethnonational and state boundaries raise the likelihood of nationalistic rivalry; (2) nationalistic rivalry disproportionately increases revisionist behavior through the nationalist mobilization of society, in contrast to other rivalries and non-rivalries; (3) nationalism motivated by transborder ethnic groups is more revisionist-prone than nationalism framed by territorial statehood, only in dyads where the former targets the latter; (4) nuclear deterrence does not reduce the probability of revisionist war within nationalistic rivalry contrary to conventional nuclear deterrence theory; and (5) joint democracy, economic interdependence, and intergovernmental organizations do not decrease revisionist behavior within nationalistic rivalry.