While it is recognized that the progenitors of war are multilayered and complex and that they consist of political, historical, religious, demographic, economic, and psycho-cultural forces, this article reports on research that looked at the role of one them, demographic trends. Demographic trends can influence the rise of ethnic territoriality and ultimately violent resolution. Two of the three protagonists in the war, namely the Bosnian Muslims and the Bosnian Serbs, are the focus of the study. The level of abstraction at which these rival ethnic groups were sensitive to demographic trends, and therefore the level at which that rivalry played out, was the local municipality, the opstina where competition over jobs and political power was manifest. It is at this level that basic tenets of ethnic competition theory are considered to have been at work. An index of ethnic competition is introduced to measure competition in terms of the relative balance of ethnic populations. This index is complemented by analysis of the trend of relative ethnic population numbers over the two decades prior to the war. Data from the Yugoslavian census show how the demographic position of the Bosnian Serbs declined dramatically in over 90% of the opstinas throughout the country. These population trends are translated into an index of demographic disadvantage. Positive correlations between the two indices (ethnic competition and demographic disadvantage) and discrete-event data of hostilities between Serbs and Muslims are found to be significant. In addition, there is some evidence that the geographic pattern of the propensity for violence, while clearly part of a broad strategic Bosnian Serb national campaign, is also associated with local prewar demographic dynamics.