The Stopping Power of Norms The Stopping Power Charli Carpenter and Alexander H. of Norms Montgomery Saturation Bombing, Civilian Immunity, and U.S. Attitudes toward the Laws of War Do Americans believe in the laws of war, and would they follow them when it really counts? A land- mark 2015 survey of public opinion on U.S. attitudes toward the laws of war suggested that Americans are relatively insensitive to even the most basic wartime taboos against the targeting of civilian populations and the use of nu- clear weapons. In the survey, Scott Sagan and Benjamin Valentino asked a sample of Americans if they would prefer a U.S. strike on a city in Iran that killed 100,000 civilians if it would end a war and save 20,000 U.S. troops’ lives. A majority of the respondents said that they would prefer such a strike, even if a nuclear weapon were used; and slightly more than two-thirds preferred the strike if it were carried out with conventional munitions. Sagan and Valentino concluded that “the majority of the U.S. public has not internalized either a be- lief in the nuclear taboo or a strong noncombatant immunity norm.”1 This conclusion, if true, is disturbing. Even in Sagan and Valentino’s “less extreme” conventional saturation-bombing scenario, survey participants were invited to contemplate—and many preferred to go along with—a horriªc war crime. Saturation bombing of undefended cities during World War II killed, by one scholarly estimate, perhaps 750,000 civilians—about seven times as many as died in the atomic bombings of Hiroshima and Nagasaki.2 Such massacres Charli Carpenter is Professor of Political Science and Legal Studies at the University of Massachusetts Amherst. Alexander H. Montgomery is Associate Professor of Political Science at Reed College. The authors are grateful to Scott Sagan and Benjamin Valentino for inspiring this project, sharing their data for replication, and providing feedback on early drafts; to Samantha Luks at YouGov for assistance with survey design and Victoria Duran for assistance with coding; and to the partici- pants at the International Studies Association workshop on Expanding the Nuclear Taboo and panel on Experimental Methods in Foreign Policy, at the University of Massachusetts Amherst’s Conºict, Violence and Security Workshop and at Stanford University’s Center for International Se- curity and Cooperation, Political Science Department, and Workshop on Nuclear Weapons and In- ternational Law for feedback on earlier versions of this article. They are especially thankful for feedback from Janina Dill, James Fearon, Peter Haas, Colin Kahl, Meredith Loken, Rose McDermott, Joseph Nye, Brian Rathbun, Kenneth Schultz, Geoffrey Wallace, and Allen Weiner. 1. Scott D. Sagan and Benjamin A. Valentino, “Revisiting Hiroshima in Iran: What Americans Really Think about Using Nuclear Weapons and Killing Noncombatants,” International Security, Vol. 42, No. 1 (Summer 2017), p. 60, doi.org/10.1162/ISEC_a_00284. 2. See data from Alexander B. Downes, Targeting Civilians in War (Ithaca, N.Y.: Cornell University Press, 2008). See also Alexander B. Downes, “Civilian Deaths from Strategic Bombing in the World War II Era,” unpublished memorandum to Charli Carpenter, 2019. The immediate deaths from Hiroshima and Nagasaki totaled 100,000–115,000. See The United States Strategic Bombing Survey: International Security, Vol. 45, No. 2 (Fall 2020), pp. 140–169, https://doi.org/10.1162/isec_a_00392 © 2020 by the President and Fellows of Harvard College and the Massachusetts Institute of Technology. 140 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/isec_a_00392 by guest on 01 October 2021 The Stopping Power of Norms 141 by air were singularly horrifying: a survivor of the Tokyo ªrebombing, who lived only because she was buried under bodies of her neighbors as they burned alive, described mothers ºeeing the inferno while the babies on their backs caught ªre. Many in Europe suffocated in cellars or were eviscerated by shrapnel in explosions. For good reason, saturation bombing, like other forms of civilian targeting, has been delegitimized as a method of war.3 Sagan and Valentino’s inference that these norms are weaker than once thought is also disturbing—particularly their conclusion about the civilian im- munity norm.4 Whereas the legality and morality of nuclear weapons use continues to be contested, it is widely believed that the prohibition on willfully targeting civilians in armed conºict is one of the oldest and most founda- tional norms of war.5 It is also now a legalized global prohibition norm, codiªed in treaty law since 1977 and considered part of customary law by the International Committee of the Red Cross.6 Thus, Sagan and Valentino’s con- clusion is a challenge not only for experimental studies of the civilian immu- The Effects of Atomic Bombs on Hiroshima and Nagasaki (Washington, D.C.: U.S. Government Printing Ofªce, June 1946). For other estimates, see Richard Overy, The Bombing War: Europe, 1939–1945 (London: Penguin, 2013). 3. Ward Thomas, The Ethics of Destruction: Norms and Force in International Relations (Ithaca, N.Y.: Cornell University Press, 2001); Tami Davis Biddle, “Strategic Bombardment: Expectation, Theory, and Practice in the Early Twentieth Century,” in Matthew Evangelista and Henry Shue, eds., The American Way of Bombing: Changing Ethical and Legal Norms, from Flying Fortresses to Drones (Ithaca, N.Y.: Cornell University Press, 2014), pp. 27–46; and Giovanni Mantilla, “Social Pressure and the Making of Wartime Civilian Protection Rules,” European Journal of International Relations, Vol. 26, No. 2 (June 2020), pp. 443–468, doi.org/10.1177/1354066119870237. 4. While Sagan and Valentino refer to “noncombatant immunity,” this term encompasses not only the civilian immunity norm but also the prohibition on willful killing of wounded or captured noncombatant soldiers (those hors de combat). Instead, we use the term “civilian immunity” to refer more restrictively to the normative principle that the civilian population speciªcally shall not be the object of attack. 5. See Fen Osler Hampson, Madness in the Multitude: Human Security and World Disorder (Don Mills, Canada: Oxford University Press, 2002); and Colm McKeogh, Innocent Civilians: The Morality of Killing in War (Basingstoke, U.K.: Palgrave Macmillan, 2002). Although powerful, this assump- tion is erroneous. See Simon Chesterman, ed., Civilians in War (Boulder, Colo.: Lynne Rienner, 2001); R. Charli Carpenter, “Innocent Women and Children”: Gender, Norms, and the Protection of Civil- ians (Aldershot, U.K.: Ashgate, 2006); and Helen M. Kinsella, The Image before the Weapon: A Critical History of the Distinction between Combatant and Civilian (Ithaca, N.Y.: Cornell University Press, 2011). 6. Article 51 of the 1977 Additional Protocol to the Geneva Conventions states, “The civilian pop- ulation as such, as well as individual civilians, shall not be the object of attack. Acts or threats of violence the primary purpose of which is to spread terror among the civilian population are pro- hibited.” See also Françoise Bouchet-Saulnier, The Practical Guide to Humanitarian Law (Lanham, Md.: Rowman & Littleªeld, 2002), 1st English-language edition, ed. and trans. by Laura Brav. The International Committee of the Red Cross (ICRC) considers this “Rule 1” among those rules considered to be customary law. See “Rule 1: The Principle of Distinction between Civilians and Combatants” (Geneva: ICRC Customary International Humanitarian Law Database, 2005), https://ihl-databases.icrc.org/customary-ihl/eng/docs/v1_rul_rule1. Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/isec_a_00392 by guest on 01 October 2021 International Security 45:2 142 nity norm speciªcally, which have tended to show that Americans strongly believe in it,7 but also for theories of norm effects and legalization in interna- tional relations.8 In this article, we revisit the strength and impact of the civilian immunity norm on American attitudes toward saturation bombing using Sagan and Valentino’s Iranian scenario. A second look is warranted because the design of their experiment makes it difªcult to know whether or not their ªnding is driven by the weakness of legal or ethical norms, by a lack of knowledge of or agreement with those norms, or by other factors such as framing effects— including Sagan and Valentino’s choice not to mention international law or norms in their survey vignette, what we term “priming by omission.” We replicated a key question in their study, where respondents were asked whether they would prefer launching the strike against the Iranian city or con- tinuing the ground war against Iran. We also introduced some variations into the survey that allow us to tease out the strength and inºuence of norms re- lative to other factors. First, we created independent measures for knowledge of, agreement with, and sensitivity to relevant ethical and legal norms. Second, we created alternative versions of the question to test whether framing effects in Sagan and Valentino’s experiment had driven their results. Across a variety of measures, we found that international ethical and legal norms against civilian targeting do exert a signiªcant constraining effect on U.S. public opinion. Accurate knowledge of international law matters, as does agreement that it is wrong to target civilians—a belief that we found to be held widely and strongly by Americans. Even being asked to think about interna- tional ethical norms prior to answering the question signiªcantly decreased support for a strike. We also found signiªcant framing effects at work in Sagan and Valentino’s experiment. In particular, requiring respondents to choose between dichoto- 7. See, for example, Adam S. Chilton, “The Laws of War and Public Opinion: An Experimental Study,” Journal of Institutional and Theoretical Economics, Vol. 171, No. 1 (March 2015), pp. 181–201, doi.org/10.1628/093245615X14188909230370; James Igoe Walsh, “Precision Weapons, Civilian Ca- sualties, and Support for the Use of Force,” Political Psychology, Vol.