The argument is tested against original cross-national data on African military history and ethnic manipulations within security institutions. The data was compiled from extensive archival research in Africa, Europe, and the United States as well as from a broad array of secondary and tertiary sources. I find that building ethnic armies against the background of a diverse officer corps, in the postcolonial context, led countries to experience nearly four times as many coup attempts as others. I also find that dismantling ethnic armies in the later period of democratization destabilized governments. When elections brought to power leaders no longer sharing the identity of the ethnic armies they inherited, coup propensity increased dramatically, from under 25% to 66%.