Center versus periphery distributional conflict is the standard explanation for separatist war. However, many separatists face strong opposition from other groups in their area. The likelihood of separatist war depends on the center’s political relationships with competing groups in the periphery. This article demonstrates two patterns in separatist war onset worldwide at the ethnic group level. Groups with a political advantage in the capital relative to their regional neighbors are less likely to have grievances about local political and economic institutions and have a lower probability of separatist war. On the other hand, ethnic groups that share territory with the most powerful ethnic group in their country are deterred from separatist violence. The center’s commitment to defend the regional status quo is particularly credible. Given the importance of within-periphery rivalries to separatist war, policy interventions designed to resolve center/periphery resource conflict may be ineffective against v...