This study explores the relationship between international intervention and the ethnification of politics and seeks to explain the degree to which international involvement affects the degree of ethnification of politics. Deriving from explanations that suggest that the ethnification of politics is attributed to the greater interethnic trust facilitated through credible institutions, this study argues that the ethnification of politics is lower in cases where the greater involvement by the international intervention in local institution-building can be observed. Using the method of structure focused comparison, the suggested hypothesis is tested on two cities in Bosnia and Hercegovina – Mostar and Sarajevo. Data was collected through a manual empirical analysis and the tracing of historical institution-building actions by the international community in each city. The main finding shows relative support for the causal relationship; however, the observed causal mechanism is different to the expected one. This signals that the degree of international involvement in local institution-building processes cannot, in isolation, explain variation on the ethnification of politics. Thus, further research is needed to both identify additional causal factors and build the interaction effects that can explain the observed variation.