Laia Balcells' book represents an important contribution to the study of violence against civilians in civil wars. Perhaps surprisingly, much of the existing literature downplays political variables and instead focuses on factors such as territorial control or organizational indiscipline. Balcells, however, develops a new theory that is inherently political.
Rivalry and Revenge is primarily interested in explaining violence against civilians in conventional (rather than irregular) civil wars. Conventional wars feature military symmetry between the two sides and have clear frontlines, behind which each side has almost complete control over the civilian population. Balcells seeks to explain why armed groups target civilians in the zones that they control. In other words, why is there violence 'behind the frontlines' in some contexts but not in others? Balcells' answer is two-fold. In the early stages of the war, prewar political mobilization is the key variable. Violence becomes more likely as the local balance of political power approaches parity because killing civilians could tip the post-war balance in one's favor. By contrast, when a locality's prewar politics overwhelmingly favor one side, the motive and/or means to change the status quo using violence is missing. In the later stages of the war, revenge becomes the key motive for violence. Individuals try to settle scores against those who killed or denounced their loved ones. This creative combination of an exogenous factor (prewar political rivalry) and an endogenous one (revenge) could, I believe, inspire much future work. A wealth of quantitative and qualitative evidence from the Spanish Civil War (and to a lesser extent, the Ivorian Civil War) supports the argument.