Chapter 7 links specific violations of Gellner’s congruence principle to ethnonationalist conflict within and between states. While it is generally accepted that violations of state-nation congruence can cause conflict, less is known about which configurations increase the risk of either civil or interstate conflict, and how these conflict types interact. Inspired by Myron Weiner’s classic model of the Macedonian Syndrome, this chapter proposes an integrated theoretical framework that links specific nationality questions to both conflict types. Using our spatial data on state borders and ethnic settlements in Europe since 1816, we show that excluded and divided groups are more likely to rebel and, where they govern on only one side of the border, to initiate territorial claims and militarized disputes. Compounding the risk of conflict, rebellion, and interstate conflict reinforce each other where ethnic division coincides with partial home rule. We obtain similar, but weaker, findings for civil wars and territorial claims in a post-1945 global sample. After World War II, governments have typically shied away from engaging in interstate disputes to address nationality questions and instead support ethnic rebels abroad.