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,

H ow strong are normative prohibitions on state behavior? We examine this question by analyzing anti-nuclear norms, sometimes called the "nuclear taboo," using an original survey experiment to evaluate American attitudes regarding nuclear use. We find that the public has only a weak aversion to using nuclear weapons and that this aversion has few characteristics of an "unthinkable" behavior or taboo. Instead, public attitudes about whether to use nuclear weapons are driven largely by consequentialist considerations of military utility. Americans' willingness to use nuclear weapons increases dramatically when nuclear weapons provide advantages over conventional weapons in destroying critical targets. Americans who oppose the use of nuclear weapons seem to do so primarily for fear of setting a negative precedent that could lead to the use of nuclear weapons by other states against the United States or its allies in the future.

Numerous polls demonstrate that U.S. public approval of President Harry Truman's decision to drop the atomic bombs on Hiroshima and Nagasaki has declined significantly since 1945. Many scholars and political figures argue that this decline constitutes compelling evidence of the emergence of a “nuclear taboo” or that the principle of noncombatant immunity has become a deeply held norm. An original survey experiment, recreating the situation that the United States faced in 1945 using a hypothetical U.S. war with Iran today, provides little support for the nuclear taboo thesis. In addition, it suggests that the U.S. public's support for the principle of noncombatant immunity is shallow and easily overcome by the pressures of war. When considering the use of nuclear weapons, the majority of Americans prioritize protecting U.S. troops and achieving American war aims, even when doing so would result in the deliberate killing of millions of foreign noncombatants. A number of individual-level traits—Republican Party identification, older age, and approval of the death penalty for convicted murderers—significantly increase support for using nuclear weapons against Iran. Women are no less willing (and, in some scenarios, more willing) than men to support nuclear weapons use. These findings highlight the limited extent to which the U.S. public has accepted the principles of just war doctrine and suggest that public opinion is unlikely to be a serious constraint on any president contemplating the use of nuclear weapons in the crucible of war.

Do the international laws of war effectively protect civilian populations from deliberate attack? In a statistical analysis of all interstate wars from 1900 to 2003 the authors find no evidence that signatories of The Hague or Geneva Conventions intentionally kill fewer civilians during war than do nonsignatories. This result holds for democratic signatories and for wars in which both sides are parties to the treaty. Nor do they find evidence that a state's regime type or the existence of ethnic or religious differences between combatants explains the variation in civilian targeting. They find strong support, however, for their theoretical framework, which suggests that combatants seek to kill enemy civilians when they believe that doing so will coerce their adversaries into early surrender or undermine their adversaries' war-related domestic production. The authors find that states fighting wars of attrition or counterinsurgency, states fighting for expansive war aims, and states fighting wars of long duration kill significantly more civilians than states in other kinds of wars.