Why do some wars result in the intentional killing of large numbers of civilians? In this article we examine the incidence of mass killing in all wars from 1945 to 2000+ In the statistical analysis of our data set of 147 wars, we find strong evidence supporting our hypothesis that mass killing is often a calculated military strategy used by regimes attempting to defeat major guerrilla insurgencies+ Unlike conventional military forces, guerrilla armies often rely directly on the local civilian population for logistical support+ Because guerrilla forces are difficult to defeat directly, governments facing major guerrilla insurgencies have strong incentives to target the guerrillas' civilian base of support+ We find that mass killing is significantly more likely during guerrilla wars than during other kinds of wars+ In addition, we find that the likelihood of mass killing among guerrilla conflicts is greatly increased when the guerrillas receive high levels of active support from the local population or when the insurgency poses a major military threat to the regime+ There is an African saying that describes the plight of civilians during times of war: "When elephants fight, the grass suffers+" Indeed, the killing of civilians is a common consequence of armed conflict+ We estimate that between thirteen and twenty-six million civilians have died in civil, international, and colonial wars since 1945+ 1 All too frequently, however, civilian deaths during war are not merely "collateral damage," as the African proverb might suggest+ In this article we argue For their helpful comments on previous versions of this article the authors thank Bear Braumoeller,

Policymakers often trumpet the potential for third parties to stop the killing associated with civil wars, yet third parties as strategic actors also have incentives to encourage longer civil wars. We argue that in order to assess the influence of third parties on civil war duration, it is necessary to consider the interdependent nature of third party interventions as they are distributed across the set of civil war combatants. We also argue that it is important to consider the geopolitical context in which civil wars occur, rather than focusing solely on characteristics internal to these conflicts. To test our hypotheses about the impact of third parties and geopolitical factors on civil war duration, we rely on event history analysis and a sample of 152 civil wars for the period 1820–1992. We find empirical support for the idea that extremely long civil wars correspond to the equitable distribution of third party interventions—stalemates prolong wars. The analysis also indicates that separatist civil wars and ongoing civil wars in states proximate to the civil war state result in civil wars of longer duration. Finally, we find that when third parties raise the stakes of the conflict by engaging in the use of militarized force against the civil war state, the duration of these conflicts is reduced. In general, our analysis underscores the importance of modeling the interdependent and dynamic aspects of third party intervention as well as the world politics of civil wars when forecasting their duration and formulating policy.

What effect do third parties have on the evolution of civil wars? The authors argue that intervention by third parties is central to the civil war process, a process that is characterized by the duration of hostilities and the type of outcome. The authors examine empirically the effect of third-party intervention into civil wars during the period 1816—1997, using the event history framework of competing risks. From the perspective of competing risks, as a civil war endures, it is at risk of experiencing a transition to one of three civil war outcomes in our sample: military victory by the government, military victory by the opposition group, and negotiated settlement. The competing risks approach provides considerably better leverage on the dynamic qualities of civil wars and, in particular, the influence of interventions by third parties. The analysis suggests that third-party interventions can be decisive in the evolution of civil wars and that third-party interventions have a different effect on the duration than different civil war outcomes. The results show that third-party intervention decreases the time until the supported group achieves military victory. Furthermore, third-party interventions, on both the government and opposition sides, increase the time until a negotiated settlement.