Why do challengers attack some states that have allies, while avoiding conflict with others? This article builds upon previous research by arguing that parity in the observable capabilities of opposing states and their allies generates greater uncertainty and miscalculations on the part of challengers, which leads to a higher probability of conflict. Unlike previous research, however, this article argues that military alliances among democracies are better able to overcome this uncertainty, making power distributions largely irrelevant. The results demonstrate that uncertainty generated at power parity is mitigated when a target state’s allies are more democratic, resulting in no overall change in the probability of conflict. This study therefore emphasizes that the effectiveness of military alliances lies not necessarily in their aggregation of power, but in their ability to co-ordinate their power and communicate this co-ordination to potential challengers.