This article looks at the effectiveness of African Union (AU) peace missions,specifica- -lly the African Union Mission in Somalia (AMISOM) and the African Union–United Nations Hybrid Operation in Darfur (UNAM- -ID). Using a comparative case study approach and based on the third-party intervention theory, it suggests that the different mission outcomes stem from the differences in mandate design and host-state cooperation. The mandate of AMISOM was relatively strong and flexible, and the cooperation of the Federal Government of Somalia was strong, which led to enabling conditions for military stabilization. UNAMID's nebulous mandate and the Sudanese government's obstructions, on the other hand, imposed severe constraints on civilian protection and territorial control. The study makes a contribution to peacekeeping scholarship through the identification of the key elements of mandate robustness and host-state cooperation as being jointly necessary, non-sufficient and offers recommendations to enhance future peacekeeping missions by the AU.