In interwar Europe, one of the most dangerous fault lines was that along which the domestic nationalisms of ethnically heterogeneous nationalizing states collided with the transborder nationalisms of neighboring “homeland” states, oriented to co-ethnics living as minorities in the nationalizing states. The clash between the nationalizing nationalism of interwar Poland and the homeland nationalisms of Germany and the Soviet Union, between the nationalizing nationalism of Czechoslovakia and the homeland nationalisms of Germany and Hungary, between the nationalizing nationalism of Romania and the homeland nationalisms of Hungary and Bulgaria – to name only a few – generated both chronic tensions and acute crises, tensions and crises that were bound up with the background to and the outbreak of the Second World War.

Analogous collisions along the same fault line threaten the stability and security of the region today. In some cases they have already led to war. As I argued in Chapter 3, the interplay between the nationalizing nationalism of Croatia and the homeland nationalism of Serbia (along with the minority nationalism of Croatia's borderland Serbs) led to the breakup of Yugoslavia. Similarly, the interplay between the nationalizing nationalism of Azerbaijan and the homeland nationalism of Armenia (initially sparked by the minority nationalism of Karabakh Armenians) led to the war over Nagorno-Karabakh. Many other collisions or potential collisions along this fault line, while they have yet to generate large-scale violence, remain potentially destabilizing. The nationalizing nationalisms of Romania and Slovakia have clashed with the homeland nationalism of Hungary.