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.For their helpful comments on previous versions of this article the authors thank Bear Braumoeller, Alex Downes, Jim Fearon, Hazem Goborah, Stathis Kalyvas, Gary King, Will Lowe, Matthew Krain, Lisa Martin, Manus Midlarsky, Bruce Russett, Nicholas Sambanis, Naunihal Singh, Abdulkader Sinno, Allan Stam, Jeremy Weinstein, and the anonymous reviewers of International Organization. We are also grateful to Wolfgang Moehler for his research assistance.Our coauthor Dylan Balch-Lindsay was killed in an automobile accident on 1 September 2002, cutting short a promising career. He was a gifted young scholar, without whom this article would not have been possible. He is sorely missed by his friends and colleagues. Donations in his name can be sent to the Dylan Balch-Lindsay Memorial Fund for Graduate Education/Foundation of the University of New Mexico, c/o Carol Brown, Department of Political Science, University of New Mexico, Albuquerque, NM, 87131-1121.