Though military interventions seem endemic in sub-Saharan Africa, more than a third of all countries have been able to avoid military coups. To solve this puzzle, this article relates the likelihood of military coups to the degree of ethnic congruence between civilian and military leaders, arguing that coup avoidance is most likely when government and army either exhibit the same ethnic bias or are both ethnically balanced. This argument is illustrated by a comparison of the diverging experiences of Zambia and Uganda. While Zambia is among Africa’s coup-free countries, Uganda’s vulnerability to military intervention has varied over time – with four coups under Obote and the Uganda National Liberation Front (UNLF) but no coups under Amin and Museveni. Drawing on original longitudinal data on the ethnic distribution of political and military posts, the article shows that the absence of military coups in Zambia goes back to the balanced composition of government and army. In Uganda, coup avoidance under Amin and Museveni can be linked to the fact that government and army exhibited the same ethnic bias, whereas the coups against the Obote and UNLF regimes reflected either ethnic incongruence between civilian and military leaders or the destabilising combination of a similarly polarised government and army.