The dissertation focuses on the internationalization of ethnic conflict by asking: Why do some ethnic conflicts lead to interstate wars while others do not? I compare the Kashmiri conflict, which escalated to two wars between India and Pakistan, to the Kurdish and Basque conflicts, which have not lead to interstate war. I conduct an in-depth historical study of all three cases to identify the possible causes that link ethnic conflicts to interstate wars. In addition to secondary sources I rely primarily on archival research and elite interviews from fieldwork conducted in Spain and Turkey. The study focuses on two interacting variables, the type of ethnic strife and the kind of external support fueling the conflict. In brief, I argue that to determine if an ethnic strife might escalate to interstate war depends on whether the conflict is a secessionist or an irredentist one, and if the kind of external support provided to the insurgents or rebel groups is a non-military or a military one.