¹ The other exception is the “inherent right of individual and collective self-defence” against an unlawful armed attack, as affirmed in Article 51. International public law draws a distinction between interventions for the sake of collective self-defense and interventions for the sake of collective security. In the former case, a third-party state comes to the defense of the victim of an armed attack; in the latter case, a third party—a state or group thereof—intervenes in an interstate conflict in order to preserve or restore international peace and security. As the 1990–1991 intervention in the Iraq-Kuwait conflict shows, the two can overlap, but they do raise distinct ethical and legal questions. See, e.g., Dinstein, Yoram, War, Aggression, and Self-Defence, 3rd ed. (Cambridge: Cambridge University Press, 2001), chaps. 9–10 CrossRefGoogle Scholar; Gray, Christine, International Law and the Use of Force, 4th ed. (Oxford: Oxford University Press, 2018), 176–98Google Scholar; Simma, Bruno et al., The Charter of the United Nations: A Commentary, 3rd ed. (Oxford: Oxford University Press, 2012)Google Scholar; Wood, Sir Michael, “Self-Defence and Collective Security: Key Distinctions,” in The Oxford Handbook of the Use of Force in International Law, ed. Weller, Marc (Oxford University Press, 2015), 649–60Google Scholar.

² Walzer, Michael, Just and Unjust Wars: A Moral Argument with Historical Illustrations, 5th ed. (New York: Basic Books, 2015)Google Scholar; Decosse, David E., ed., But Was It Just? Reflections on the Morality of the Persian Gulf War (New York: Doubleday, 1992)Google Scholar; McMahan, Jeff and McKim, Robert, “The Just War and the Gulf War,” Canadian Journal of Philosophy 23, no. 4 (1993): 501–41CrossRefGoogle Scholar. Widely cited monographs, all of which are virtually silent on interstate conflicts, include, in chronological order, Rodin, David, War and Self-Defense (Oxford: Clarendon Press, 2002)CrossRefGoogle Scholar; McMahan, Jeff, Killing in War (Oxford: Oxford University Press, 2009)CrossRefGoogle Scholar; Fabre, Cécile, Cosmopolitan War (Oxford: Oxford University Press, 2012)CrossRefGoogle Scholar; Frowe, Helen, Defensive Killing (Oxford: Oxford University Press, 2014)CrossRefGoogle Scholar; Frowe, Helen, The Ethics of War and Peace, 2nd ed. (London: Routledge, 2016)Google Scholar; Draper, Kai, War and Individual Rights: The Foundations of Just War Theory (Oxford: Oxford University Press, 2016)Google Scholar; Buchanan, Allen, Institutionalizing the Just War (Oxford: Oxford University Press, 2017)Google Scholar; Haque, Adil Ahmad, Law and Morality at War (Oxford: Oxford University Press, 2017)CrossRefGoogle Scholar; Benbaji, Yitzhak and Statman, Daniel, War by Agreement (Oxford: Oxford University Press, 2019)CrossRefGoogle Scholar; Tadros, Victor, To Do, To Die, To Reason Why: Individual Ethics in War (Oxford University Press, 2020)CrossRefGoogle Scholar; Ripstein, Arthur et al., Rules for Wrongdoers: Law, Morality, War (Oxford: Oxford University Press, 2021)CrossRefGoogle Scholar; Ripstein, Arthur, Kant and the Law of War (Oxford: Oxford University Press, 2021)CrossRefGoogle Scholar. Two recent collections of essays on the morality of war that do not have a single chapter on this issue either are Fabre, Cécile and Lazar, Seth, eds., The Morality of Defensive War (Oxford: Oxford University Press, 2014)CrossRefGoogle Scholar; Lazar, Seth and Frowe, Helen, eds., The Oxford Handbook of Ethics of War (Oxford: Oxford University Press, 2018)CrossRefGoogle Scholar.

³ Pinker, Steven, The Better Angels of Our Nature: A History of Violence and Humanity (New York: Penguin Books, 2011)Google Scholar. For a powerful critique of this view, see Bear F. Braumoeller, Only the Dead (Oxford: Oxford University Press, 2019).

⁴ A technical point: throughout this essay, when I say that some agent is justified in embarking on a given course of action, I mean that the facts are such as to warrant that course and that she has evidence to believe that so are the facts.

⁵ Buchanan, Allen and Keohane, Robert O., “The Preventive Use of Force: A Cosmopolitan Institutional Proposal,” Ethics & International Affairs 18, no. 1 (2004): 1–22 CrossRefGoogle Scholar.

⁶ On alternatives to military force, see, e.g., Fabre, Cécile, Economic Statecraft: Human Rights, Sanctions, and Conditionality (Cambridge, MA: Harvard University Press, 2018)Google Scholar; Pattison, James, The Alternatives to War: From Sanctions to Nonviolence (Oxford: Oxford University Press, 2018)Google Scholar.

⁷ I do not address the question of whether intervention is morally mandatory. For the view that any war that is permissible is also mandatory, see Oberman, Kieran The Myth of the Optional War: Why States Are Required to Wage the Wars They Are Permitted to Wage,” Philosophy & Public Affairs 43, no. 4 (2015): 255–86.CrossRefGoogle Scholar

⁸ On the relevance of consent for justified defensive action, see Parry, Jonathan, “Defensive Harm, Consent, and Intervention,” Philosophy & Public Affairs 45, no. 4 (2017): 356–96CrossRefGoogle Scholar.

⁹ Walzer, Just and Unjust Wars, 59. See also Rodin, War and Self-Defense. Put differently, on this moralized account of global crises, a conflict is a global crisis by dint of the fact that it results from the breach of a norm we all endorse. I adopt a nonmoralized account, whereby a conflict is a global crisis by dint of the harms it causes. Thanks to Allen Buchanan for this suggestion. Incidentally, even on a nonmoralized account, it does not follow from the claim that a breach of the universal norm against aggression is a global crisis, that resorting to military action in defense of that norm is morally justified.

¹⁰ Dinstein, War, Aggression, and Self-Defence, 225.

¹¹ See also Cécile Fabre, “Cosmopolitanism and Wars of Self-Defence,” in The Morality of Defensive War, ed. Fabre and Lazar, 90–114.

¹² As Allen Buchanan pointed out to me, whether a crisis is construed as a global crisis is partly contingent on our awareness of it and our sensibilities. If we do not know that atrocities are taking place on a large scale somewhere, though we feel their effects through, e.g., population displacements, we will not regard those atrocities as a global (or regional) crisis.

¹³ For lack of space, I focus on cases in which the threat originates with Aggressor. Note that by preventing an attack, I mean eliminating it before it materializes, for example, by destroying the enemy’s air force. That is not the same as deterring it.

¹⁴ See, e.g., Ripstein, Rules for Wrongdoers; Benbaji and Statman, War by Agreement, chap. 3. By “the orthodox view,” I mean the view of war that has been dominant since the end of the nineteenth century. In medieval, early modern, and modern accounts, the prevention of attacks, the punishment of wrongdoers, and the recovery of wrongfully taken property or territory were deemed just causes for war. See Neff, Stephen C., War and the Law of Nations: A General History (Cambridge, UK: Cambridge University Press, 2005)CrossRefGoogle Scholar.

¹⁵ For the view that, under certain conditions, rights to the basic necessities of life may justifiably be defended by force, see Grotius, Hugo, The Rights of War and Peace, ed. Tuck, Richard (1625; repr., Indianapolis, IN: Liberty Fund, 2005)Google Scholar, II.II; Luban, David, “Just War and Human Rights,” Philosophy & Public Affairs 9, no. 2 (1980): 160–81Google Scholar; Fabre, Cosmopolitan War, chap. 3.; Lippert-Rasmussen, Kasper, “Pogge, Poverty, and War,” Politics, Philosophy & Economics 16, no. 4 (2017): 446–69CrossRefGoogle Scholar. On preventive wars, see, e.g., Luban, David Preventive War,” Philosophy & Public Affairs 32, no. 3 (2004): 207–48CrossRefGoogle Scholar; Rodin, David and Shue, Henry, eds., Preemption: Military Action and Moral Justification (Oxford: Oxford University Press, 2007)Google Scholar; Chatterjee, Deen K., ed., The Ethics of Preventive War (Cambridge, UK: Cambridge University Press, 2013)CrossRefGoogle Scholar; Buchanan, Institutionalizing the Just War, chap. 7.

¹⁶ Stueck, William, “The United Nations, the Security Council, and the Korean War,” in The United Nations Security Council and War, ed. Lowe, Vaughan et al. (Oxford: Oxford University Press, 2008), 268 Google Scholar; James Cockayne and David M. Malone, “The Security Council and the 1991 and 2003 Wars in Iraq,” in The United Nations Security Council and War, ed. Lowe et al., 386.

¹⁷ See Kavka, Gregory S., Moral Paradoxes of Nuclear Deterrence (Cambridge, UK: Cambridge University Press, 1987)Google Scholar; Shue, Henry, ed., Nuclear Deterrence and Moral Restraint: Critical Choices for American Strategy (Cambridge, UK: Cambridge University Press, 1989)CrossRefGoogle Scholar; McMahan, Jeff, “Deterrence and Deontology,” Ethics 95, no. 3 (1985): 517–36CrossRefGoogle Scholar; Goodin, Robert E., “Nuclear Disarmament as a Moral Certainty,” Ethics 95, no. 3 (1985): 641–58CrossRefGoogle Scholar; Wasserstrom, Richard, “War, Nuclear War, and Nuclear Deterrence: Some Conceptual and Moral Issues,” Ethics 95, no. 3 (1985): 424–44CrossRefGoogle Scholar; Dworkin, Gerald, “Nuclear Intentions,” Ethics 95, no. 3 (1985): 445–60CrossRefGoogle Scholar; Lee, Steven, Morality, Prudence, and Nuclear Weapons (Cambridge, UK: Cambridge University Press, 1993)CrossRefGoogle Scholar. For a brief reference to deterrence as a justification for punitive war, see Luban, David War as Punishment,” Philosophy & Public Affairs 39, no. 4 (2011): 299–330 CrossRefGoogle Scholar. Luban’s critical discussion of punitive wars takes a retributivist as opposed to a deterrence-based conception of punishment.

¹⁸ For a similar argument in the context of nuclear deterrence, see Wasserstrom, “War, Nuclear War, and Nuclear Deterrence,” 437–38. Suppose that Intervener kills all liable individuals within Aggressor. It is possible, of course, that there would remain nonliable members of Aggressor left alive, who could be deterred from pursuing Aggressor’s unjust policy at t3. As Allen Buchanan pointed out to me, we can construe the Allies’ destructive policy toward Germany during World War II as attempting to deter successive generations of German citizens and leaders from ever waging a war of aggression. In this case, however, Intervener’s use of force would be a case of general, not special, deterrence. I address this issue below. I lack the space, however, to address the permissibility of “in perpetuity” multigenerational deterrence.

¹⁹ Thanks to Gideon Elford for suggesting this point. To be clear: I mean that Intervener is justified in resorting to F, under those conditions, as a deterrent. Even if F does not deter Aggressor, it might eliminate it, and might be justified on those grounds.

²⁰ See, e.g., Kenny, Anthony, The Logic of Deterrence (Chicago, IL: University of Chicago Press, 1985)Google Scholar.

²¹ McMahan, “Deterrence and Deontology”; Kavka, Moral Paradoxes of Nuclear Deterrence, esp. chaps. 1–2. Sometimes, of course, the effects of a threat are such as to render the threat impermissible. For example, we can imagine cases in which the issuing of a threat poses an unacceptably high risk of an accidental military confrontation between Aggressor and Intervener, with far worse and disproportionately harmful consequences for international peace and security. I am grateful to Elad Uzan for this point.

²² On the morality of extrication, see Cecil Anthony John Coady, “Escaping from the Bomb: Immoral Deterrence and the Problem of Extrication,” in Nuclear Deterrence and Moral Restraint, ed. Shue, 163–226. Thanks to Gideon Elford for raising the problem of moral hazard.

²³ See also Hendrickson, David C., “The Ethics of Collective Security,” Ethics & International Affairs 7, no. 1 (1993): 1–15 CrossRefGoogle Scholar.

²⁴ See also McMahan, Jeff, “Just Cause for War,” Ethics & International Affairs 19, no. 3 (2005): 1–21 CrossRefGoogle Scholar, esp. 16.

²⁵ By contrast, a regime’s failure to punish one single instance of murder within its borders is unlikely to provide incentives to future murderers. Thanks to an anonymous reviewer for Social Philosophy & Policy for this point.

²⁶ Tadros, Victor, The Ends of Harm: The Moral Foundations of Criminal Law (Oxford: Oxford University Press, 2011.)CrossRefGoogle Scholar

²⁷ Victor Tadros, Wrongs and Crimes (Oxford: Oxford University Press, 2016), 65–66. I am reconstructing a “Tadrosean” argument for intervention. I do not know whether Tadros would endorse it. I myself once endorsed the redemption obligation argument for deterrence, in the context of economic sanctions; see Fabre, Economic Statecraft, 52–54. I now think that this was a mistake. For a sophisticated study of the problems raised by Tadros’s treatment of the innocent in general, see, e.g., Tomlin, Patrick, “Innocence Lost: A Problem for Punishment as Duty,” Law and Philosophy 36, no. 3 (2017): 225–54CrossRefGoogle Scholar.

²⁸ On the interplay between conventional and nuclear deterrence, see Lee, Morality, Prudence, and Nuclear Weapons, chaps. 4, 6.

²⁹ It does not show either that other justifications for intervention, such as simply thwarting Aggressor’s attack, are inapt; deterrence is not the only justification for resorting to war. That said, the concerns that deterrence raises, such as the cataclysmic consequences of a military confrontation between great powers, are likely to apply to any such confrontation, however it is justified.

³⁰ Walzer, Just and Unjust Wars, chap. 15.

³¹ Thanks to Paul Tucker for pressing me on this point.

³² Buchanan, Institutionalizing the Just War, esp. chaps. 1, 2, 7.

³³ See, e.g., Sarooshi, Danesh, The United Nations and the Development of Collective Security (Oxford: Clarendon Press, 1999)Google Scholar; Lowe, Vaughan et al., eds., The United Nations Security Council and War; Alexander Orakhelashvili, Collective Security (Oxford: Oxford University Press, 2011)Google Scholar; Weller, Marc, ed., The Oxford Handbook of the Use of Force in International Law (Oxford: Oxford University Press, 2015)CrossRefGoogle Scholar.

³⁴ GA Res. 377 (V) (1950), para. A. See Dominik Zaum, “The Security Council, the General Assembly, and War: The Uniting for Peace Resolution,” in The United Nations Security Council and War, ed. Lowe et al., 154–74.

³⁵ On UNSC paralysis during the Cold War, see, e.g., Hurrell, Andrew, “Collective Security and International Order Revisited,” International Relations 11, no. 1 (1992): 37–55 CrossRefGoogle Scholar; Gray, International Law and the Use of Force, chap. 6; Ian Johnstone, “When the Security Council Is Divided: Imprecise Authorizations, Implied Mandates, and the ‘Unreasonable Veto’,” in The Oxford Handbook of the Use of Force in International Law, ed. Weller, 227–50. On the difficulties that beset attempts at reforming the UNSC, see Luck, Edward C., “Principal Organs,” in The Oxford Handbook on the United Nations, ed. Weiss, Thomas G. and Daws, Sam (Oxford: Oxford University Press, 2007), 653–74Google Scholar.

³⁶ See Buchanan, Institutionalizing the Just War, esp. chap. 3.

³⁷ Buchanan, Institutionalizing the Just War, chaps. 1, 7; Buchanan and Keohane, “The Preventive Use of Force.” Theirs is a comprehensive set of institutional proposals for the use of preventive force and for humanitarian intervention to overthrow tyrannical regimes. Some of their proposals, notably the suggestion that democratic states should commit themselves to agree to outside intervention if they descend into authoritarian strife, are not relevant to this essay. For lack of space, I set aside the difficult question of how such reforms might be brought about, whether, for example, they might be brought about by illegal acts. See Buchanan, Allen, “From Nuremburg to Kosovo: The Morality of Illegal International Legal Reform,” Ethics 111, no. 4 (2001): 673–705 CrossRefGoogle Scholar. The main differences between their proposal and my own are the following: I hold on to the UNSC monopoly on the resort to force; I vest the determination of whether force is warranted or not to a separate body; and I propose subjecting failures to intervene to ex post scrutiny.

³⁸ For a state of the art review of the “Third UN,” see Wapner, Paul, “Civil Society,” in The Oxford Handbook on the United Nations, ed. Weiss, and Daws, , 254–63Google Scholar. Buchanan advocates including human rights organizations in whichever body decides to use force, as distinct from, in my proposal, whichever body declares that force is warranted. See Buchanan, Institutionalizing the Just War, 42.