Ethnic stacking, or recruiting and promoting soldiers based on shared identity, is a prolific form of coup proofing in autocratic regimes. Leaders attempt to ensure the loyalty of security institutions through a triple bind of ethnic affinity, ethnically-based patronage, and publicly tied fates. Yet, the process of building a coethnic army can backfire as it both discriminates against existing out-group officers and provokes tensions between sub-groups—igniting preemptive coups and competition for control over security institutions. This article leverages new data on ethnic stacking practices in Africa to conduct the first large-N analysis of how ascriptive coup proofing impacts leader tenure. I find that ethnically restructuring the armed forces does indeed create a strong backlash, with coup risk increasing in autocracies by nearly 200% over the first three years of such efforts. However, ethnic stacking still significantly extends leader tenure (by around 65%), in part by decreasing the long-term risk of coup attempts (by over 30%). Thus, if autocratic leaders can survive the process of building an ethnic army, they can obviate threats emanating from within the military and deeply entrench their rule.