Armed conflicts as distant as those in Sri Lanka and Northern Ireland have been fueled by co-ethnic supporters of the combatants. Yet civil wars also draw in third parties who are not related to the combatants: opposing US and Soviet interventions in southern Africa and Central America provide several prominent examples. While we know that biased interventions in civil wars have different effects that neutral efforts, does it matter who intervenes and the tools they employ? Rebels organized along ethnic lines often receive sustained support from sympathetic co-ethnics abroad. This support is distinct from that provided by unrelated supporters. In this paper, I show that civil wars last especially long when co-ethnics abroad intervene. Co-ethnics, or ethnic kin, increase rebels’ capabilities when they provide sustained and reliable support. Military, financial, or logistic support from co-ethnics who often feel deep sympathy, guilt, or responsibility for their kin’s welfare helps rebels to continue fighting long after they otherwise would be overwhelmed. This support also can encourage rebels to continue fighting to exact a better deal, or it can complicate bargaining, stymieing a settlement. External support also increases an insurgency’s viability and facilitates recruiting. However, it rarely is enough to ensure victory. Thus, it simply prolongs conflicts. By focusing on the role of third parties in ethnic civil wars, this paper answers three central puzzles. First, why do some civil wars last longer than others? I demonstrate how support from different groups of third parties prolongs wars. Second, when and how does ethnicity affect the length of civil wars? I answer this question by elaborating the conditions under which ethnicity shapes the support available to combatants. Third, why does the existing literature offer contradictory answers to this second question? By focusing on the role of third party support as a mechanism linking ethnicity and conflict duration, I answer questions about ethnic civil wars that previous research has not been able to resolve. In the next section, I review existing literature on the role of ethnicity in civil wars. I then develop my theory, which focuses on the ways that different types of third parties intervene on behalf of combatants in civil wars. I then turn to the results of the analysis, which demonstrates that these interventions profoundly affect the length of civil wars, but that different actors produce very different results. I conclude with a brief review of the main findings and a discussion of the policy implications of the study
