Administrative boundaries play a crucial but understudied role in managing separatist conflict in federal and decentralized states. I argue that closely aligning these boundaries with ethnic settlement patterns helps sustainably defuse separatist conflicts. When minorities control unified ethnic regions, autonomy enables them to pursue their policy goals, reducing grievances. Conversely, when they are fragmented across regions or remain minorities at the regional level, autonomy is more likely to violate their aspirations and fuel continued separatist mobilization. I test this argument using new spatial data on regional boundaries in 132 multi-ethnic countries (1946–2017). A difference-in-differences design exploiting within-group variation over time reveals that administrative boundaries critically shape separatist mobilization. The step-wise implications of my hypothesized mechanisms are reflected in analyses of intermediate outcomes and in influential case accounts. These findings underscore the importance of regional boundary design in managing ethnic conflict and offer insights for institutional responses to separatist pressures.
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