Why do non-state groups engage in armed conflict with each other? Most studies on internal conflict focus on the dyadic interaction between the state and a rebel group, providing less attention to inter-group fighting. In an attempt to contribute to the limited body of quantitative research on non-state violence, I argue that the opportunity structures and security problems created by weak state institutions may help explain the occurrence of violence between groups. Drawing on the argument that state capacity is important for domestic peace, I claim that non-state violence is more likely when groups are forced to provide for their own security within the state sphere. However, although weak state capacity structures create opportunities for groups to engage in fighting, an anarchical environment in itself might not explain inter-group violence. Thus, I argue that the interplay between economic and political exclusion of groups and weak state capacity further increases the risk of non-state conflict, creating both opportunities and motivation to engage in conflict. Whereas the majority of quantitative studies that focus on marginalization emphasize rebel groups in relation to the state, I argue that violence to ameliorate uneven distribution is just as likely to be directed at non-state groups who receive a larger share of economic and political welfare.
Utilizing data on Sub-Saharan Africa from 1989 to 2011, I conduct a quantitative analysis studying the effect of weak state capacity on inter-group violence. The empirical analysis provides support for the theoretical expectation that weak state capacity increases the risk of inter-group violence, and that the combined presence of economic marginalization and weak state capacity further increases this risk. However, I find no support for a combined effect of weak state structures and political marginalization. The findings highlight the importance of state capacity for internal peace, and lend support to the literature emphasizing the relationship between state strength and conflict. Also, they illustrate the advantages of a more uniform theoretical framework, focusing on a specific type of violence between organized groups.