¹ Statute of the International Criminal Court, 2187 UNTS 90, Arts. 8(2)(b)(i), (iv), (xx), and 8(2)(e)(i) (hereinafter, Rome Statute) although many other war crimes can ultimately be traced to the violation of one or more of these principles.

² See, e.g., M. Walzer, Just and Unjust Wars (1992).

³ J. McMahan, Killing in War (2009); D. Rodin, War and Self-Defense (2002); and C. Fabre, Cosmopolitan War (2012).

⁴ A point of clarification is in order here. I will not provide an account of when or why a particular moral wrong should be criminalized. This is a thorny question in domestic criminal law theory which does not seem to be critical to our enquiry here, given that most of the grave wrongs we shall consider certainly satisfy the requirements for criminalization under almost any existing approach – the question, by contrast, is about the conditions under which they should be criminalized.

⁵ See Section 2, infra.

⁶ See, generally, M.L. Gross, Moral Dilemmas of Modern War: Torture, Assassination, and Blackmail in an Age of Asymmetric Conflict (2010).

⁷ See Section 5, infra.

⁸ See Chehtman, A., ‘Jurisdiction’, in Dubber, M.D. and Hörnle, T. (eds.), The Oxford Handbook of Criminal Law (2014), 399 Google Scholar.

⁹ See Section 3, infra.

¹⁰ See, e.g., McMahan, supra note 3.

¹¹ This is subject to a few qualifications. Some of those fighting an unjust war whose responsibility is greatly mitigated by ignorance or duress, and who will contribute very little or nothing at all to the unjust aims, would not be liable to being killed. By contrast, some of those fighting with just cause, but who do so by unjust means, or exceed their right to defend themselves, may also become liable. I will not address this further issue here.

¹² I follow here Fabre, supra note 3. See, similarly, Hurka, T., ‘Proportionality in the Morality of War’, (2004) 33 (1) Philosophy and Public Affairs 34 CrossRefGoogle Scholar, Section 1; McMahan, supra note 3.

¹³ McMahan, supra note 3, at 203 and accompanying footnotes.

¹⁴ See, e.g., C. Fabre, Cosmopolitan Peace (2016), 7.3.

¹⁵ Ryan, C., ‘Democratic Duty and the Moral Dilemmas of Soldiers’, 2011 122 (1) Ethics 10, at 15CrossRefGoogle Scholar.

¹⁶ See, e.g., S. Neff, War and the Law of Nations (2005), 63, citing F. Suárez, A Work on the Three Theological Virtues: Faith, Hope, and Charity (1958), 813.

¹⁷ See Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion of 8 July 1996, [1996] ICJ Rep. 226, para. 78; Protocol Additional to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of International Armed Conflicts (Protocol I), 8 June 1977, 11 UNTS 3, Arts. 48, 51(2), and 52(2) (hereinafter, Additional Protocol I or AP I); Protocol Additional to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of Non-International Armed Conflicts (Protocol II), 8 June 1977, 1125 UNTS 609, Art. 13(2) (hereinafter, Additional Protocol II or AP II).

¹⁸ McMahan, J., ‘The Basis of Moral Liability to Defensive Killing’, (2005) 15 (1) Philosophical Issues, 386 CrossRefGoogle Scholar.

¹⁹ Arguably, there are exceptions to this general position (as when an individual fighting in an otherwise objectively unjust war uses force to justifiedly prevent a specific wrongful attack by someone otherwise fighting an objectively just war, or when a war is justified exclusively on lesser evil grounds), but they are not particularly relevant for our purposes.

²⁰ See Art. 4 of the Lieber Code (Instructions for the Government of Armies of the United States in the Field), General Order No. 100, Art. 14 (24 April 1863). For a book-length treatment of this principle, see L. May and J. Ohlin, Necessity in International Law (2014). For a more demanding account, see Beer, Y., ‘Humanity Considerations Cannot Reduce War's Hazards Alone: Revitalizing the Concept of Military Necessity’, (2015) 26 (4) European Journal of International Law 801 CrossRefGoogle Scholar. For a leading philosophical account, see Lazar, S., ‘Necessity in Self-Defense and War’, (2012) 40 (1) Philosophy and Public Affairs 3 CrossRefGoogle Scholar.

²¹ McMahan, J., ‘War Crimes and Immoral Action in War’, in Duff, A. et al. (eds.), The Constitution of Criminal Law (2013), 151, at 160CrossRefGoogle Scholar.

²² See Additional Protocol I, Art. 51(5)(b). See also McMahan, J., ‘Proportionate Defense’, (2014) 23 Journal of Transnational Law & Policy 1, at 6–7Google Scholar.

²³ The principle of proportionality in attack is codified in Art. 51(5)(b) of AP I, and repeated in Art. 57. ‘While Additional Protocol II does not contain an explicit reference to the principle of proportionality in attack, it has been argued that it is inherent in the principle of humanity which was explicitly made applicable to the Protocol in its preamble and that, as a result, the principle of proportionality cannot be ignored in the application of the Protocol.’ J-M. Henckaerts and L. Doswald-Beck, Customary International Humanitarian Law (2005), Vol I: Rules, at 48.

²⁴ Except for the one of legitimate authority, which is independent of the requirement of just cause, but which is contested by some revisionist just war theorists.

²⁵ Fabre, supra note 14, at 7.3 in fine.

²⁶ Ibid.

²⁷ Ibid., at 7.2.2.

²⁸ See A. Chehtman, The Philosophical Foundations of Extraterritorial Punishment (2010), Ch. 3.

²⁹ Fabre, supra note 14, at 7.2.2. I have advocated a similar justification in note 28, supra, in Chapter 2. For seminal articulations, see Feinberg, J., ‘The expressive function of Punishment’, in Feinberg, J., Doing & Deserving: Essays in the Theory of Responsibility (1974)Google Scholar; A. Duff, Punishment, Communication, and Community (2001).

³⁰ Fabre, supra note 14, at 7.4.1.

³¹ Ibid.

³² Ibid.

³³ Ibid., at 7.4.2.

³⁴ Ibid.

³⁵ See Walzer, supra note 2, at 312.

³⁶ See, e.g., A. Ashworth, Principles of Criminal Law (2009), Ch. 5.

³⁷ I believe this is true both as a matter of normative theory and as a matter of law. I deal here with the former issue, and with superior orders in international law in Section 5, infra.

³⁸ Fabre, supra note 14, at 7.4.2.

³⁹ Although I exemplify this position by reference to a reasonable mistake of fact caused by the intervention of a legitimate authority, the same argument would apply, under certain circumstances, to mistakes of law, although this would be rarer. This has been recently discussed by the Israeli Supreme Court in HCJ 1971/15 Al-Masri v. The Chief Military Advocate General. Yet, this issue is beyond the scope of this article.

⁴⁰ de Vitoria, F., ‘On the American Indians’, in Pagden, A. and Lawrance, J. (eds.), Political Writings (1991) 231, at 282–3Google Scholar. Grotius reached a similar conclusion, holding that situations of invincible doubt sufficed to place both belligerents in a somewhat symmetrical position. See Neff, supra note 16, at 99.

⁴¹ For different versions of this argument, see Estlund, D., ‘On Following Orders in an Unjust War’, (2007) 15 (2) The Journal of Political Philosophy 213 CrossRefGoogle Scholar; Ryan, supra note 15; and most recently M. Renzo, ‘Duties of Citizenship and Just War’ (unpublished typescript on file with the author).

⁴² See J. Raz, The Morality of Freedom (1988), 53; Raz, J, ‘The Problem of Authority’, (2006) 90 Minnesota Law Review 1003, at 1014Google Scholar. I use Raz for simplicity here. My argument would also work, albeit to different degrees, with many of the other contemporary accounts of legitimate authority available in the literature.

⁴³ Raz argues that authoritative directives pre-empt individual decision-making; Raz, The Morality of Freedom, supra note 42, at 46. I need not take such a demanding notion of authoritative directive. I believe that Raz's conceptual understanding of authority is compatible with a slightly less rigid notion of protected reasons, such as prima facie or pro tanto reasons for action. For a recent defence of this view see Renzo, supra note 41, App.

⁴⁴ Raz, The Morality of Freedom, supra note 42, at 47–8.

⁴⁵ Ryan, supra note 15, at 30, 32.

⁴⁶ Estlund, supra note 41, at 232.

⁴⁷ Renzo, supra note 41, at 17.

⁴⁸ J. Raz, Practical Reasons and Norms (1999), 47.

⁴⁹ Raz, The Morality of Freedom, supra note 42, at 62.

⁵⁰ J. Greene, Moral Tribes: Emotion, Reason and the Gap Between Us and Them (2013), Ch. 3.

⁵¹ Ibid.

⁵² Sunstein, C., ‘The Law of Group Polarization’, (2002) 10 (2) The Journal of Political Philosophy 175 CrossRefGoogle Scholar.

⁵³ Greene, supra note 50. See also Sunstein, C. and Vermeule, A., ‘Conspiracy Theories: Causes and Cures’, (2009) 17 (2) The Journal of Political Philosophy 202 CrossRefGoogle Scholar.

⁵⁴ Greene, supra note 50.

⁵⁵ Ibid.

⁵⁶ Neff, supra note 16, at 57 (and its references in footnote 62).

⁵⁷ de Vitoria, F., ‘On the law of war’, in Pagden, A. and Lawrence, J. (eds.), Political Writings (1991), 293 at 307 Google Scholar, cited in Estlund, supra note 41, at 214.

⁵⁸ As I indicated in Section 2, supra, that such war systematically violates in bello rules suffices to turn it manifestly unjust or morally abhorrent.

⁵⁹ It may be argued that this is compatible with their moral responsibility being mitigated in certain cases by situations of extreme duress. Yet this qualification would imply that they are liable to being punished, and would affect the extent of that liability. For a good discussion of the difference between justification and total excuse, see López, E.R., ‘Can there be Full Excuses for Morally Wrong Actions?’, (2006) 73 (1) Philosophy and Phenomenological Research 124 CrossRefGoogle Scholar.

⁶⁰ It is interesting to point out that at the level of the deep morality of war he is sympathetic to the claim that acts of almost every type of participant in an objectively unjustified war are akin to war crimes. See McMahan, supra note 21. The problem is, as I have suggested, that such revisionist position would be implausibly over-inclusive.

⁶¹ McMahan, supra note 21, at 177.

⁶² Ibid., at 172.

⁶³ Admittedly, it is difficult to find historical examples of belligerents fighting a manifestly unjust war exclusively which did not also systematically violate in bello rules. But this is only because belligerents manifestly violating ad bellum principles also tend to systematically violate in bello rules. Soviet military action in Hungary in 1956 may be a plausible example of the former. See C. Gati, Failed Illusions: Moscow, Washington, Budapest, and the 1956 Hungarian Revolt (2006).

⁶⁴ For example, Baez et al., argue that in the Colombian conflict a majority of former fighters ‘joined paramilitary groups for economic reasons’, and only 13 per cent had an ideological motivation for joining. See Baez, S. et al., ‘Outcome-oriented moral evaluation in terrorists’, (2017) 1 Nature Human Behavior 4 Google Scholar. Yet, see also Á. Gómez et al., ‘The devoted actor's will to fight and the spiritual dimension of human conflict’, Nature Human Behavior, available at www.nature.com/articles/s41562-017-0193-3.pdf.

⁶⁵ McMahan, supra note 21.

⁶⁶ See Section 3, supra, and Chehtman, supra note 28, at 43–6.

⁶⁷ See Jo, H. and Simmons, B., ‘Can the International Criminal Court Deter Atrocity?’, (2016) 70 (3) International Organization 443 CrossRefGoogle Scholar.

⁶⁸ E.g., Prosecutor v. Erdemovic, Judgement, Case No. IT-96-22-A, A. Ch., 7 October 1997.

⁶⁹ McMahan, supra note 21, at 173.

⁷⁰ See, e.g., D. Reiter and A.C. Stam, Democracies at War (2002), 65–9.

⁷¹ See, e.g., J. Elster, Closing the Books: Transitional Justice in Historical Perspective (2004).

⁷² Additional Protocol II.

⁷³ McMahan, supra note 21, at 173.

⁷⁴ Ibid.

⁷⁵ See, e.g., Koskenniemi, M., ‘Between Impunity and Show Trials’, (2002) 6 Max Planck Yearbook of United Nations Law 1 CrossRefGoogle Scholar.

⁷⁶ See, e.g., G. Simpson, Law, War & Crime (2007), 16.

⁷⁷ McMahan, supra note 21, at 173.

⁷⁸ I assume for present purposes a rough overlap between manifestly unjust and manifestly unlawful wars. This is an important and sensitive assumption, which is beyond the scope of this article.

⁷⁹ It assumes, of course, they would satisfy other conditions such as mental capacity. Indeed, this argument does not necessarily include child soldiers or other analogous cases.

⁸⁰ Whether they should be punished or not and what kind of penalties should be imposed on them largely depends on the specifics of the situation. I am happy to concede that other considerations would be relevant to determine what to do, such as the prospects of a sustainable peace, the needs for reconstruction, and so on, and that in a significant number of cases it would be counterproductive to punish them.

⁸¹ Art. 8bis (1), Res. RC/Res. 6, The crime of aggression, Annex I, adopted on 11 June 2010. This definition, specifically the requirement that the violation of ad bellum rules be ‘manifest’, remains controversial.

⁸² Rome Statute, Art. 33(1)(c).

⁸³ See references in Y. Dinstein, The Defence of ‘Obedience to Superior Orders’ in International Law (1965) and, more recently, M. Osiel, Obeying Orders: Atrocities, Military Discipline and the Law of War (1999).

⁸⁴ Gaeta highlights that the British, French and Russian prosecutors in Nuremberg argued that this defence was not possible due to the manifest illegality of the orders, rather than simply relying on Article 8 of the London Charter. Gaeta, P., ‘The Defence of Superior Orders: The Statute of the International Criminal Court versus Customary International Law’, (1999) 10 European Journal of International Law 172, at 180 (emphasis added)CrossRefGoogle Scholar. Similarly, Garraway argues that the drafting history of the Nuremberg Charter at several points included a provision on superior orders, and suggests, quoting General Nikitchenko, that the reason they were not considered a valid defence in that context was that those particular defendants could not be carrying out orders of a superior. Garraway, C., ‘Superior orders and the International Criminal Court: Justice delivered or justice denied’, (1990) 81 (836) International Review of the Red Cross 785, at 786Google Scholar. Put differently, Nuremberg was in some way a rejection of the traditional position on the defence of superior orders which, as Oppenheim wrote in the first edition of his book, stated that ‘[i]n case members of forces commit violations ordered by their commanders, the members may not be punished, for the commanders are alone responsible, and the latter may, therefore, be punished as war criminals on their capture by the enemy’. Cited in Garraway, at 786. Consistent with this, in the High Command case (11 NMT, at 506) the judges acquitted the defendants on the basis that orders were not manifestly unlawful or known to be unlawful (even though Control Council Law No. 10 followed the IMT Statute on the rejection of superior orders).

⁸⁵ Gaeta, supra note 84, at 191.

⁸⁶ Walzer recognizes that the defence of superior orders can be based on ignorance. Walzer, supra, note 2, at 312. Similarly, Dinstein argues that the superior orders which are not manifestly unlawful affect individual awareness of the illegality of an act. Dinstein, supra, note 83, at 27–8. More in line with the proposal defended here, Cryer suggests that ‘the manifest illegality test is a way of determining if the defendant ought to have known that the order was illegal’. Cryer, R., ‘Superior Scholarship on Superior Orders’, 2011 Journal of International Criminal Justice 959, at 963Google Scholar.

⁸⁷ Rome Statute, Art. 33(1)(c).

⁸⁸ See, e.g., the contributions to E. Wilmshurst (ed.), International Law and the Classification of Conflicts (2012), especially those in Part II.

⁸⁹ Although they can be detained for the duration of the hostilities, this detention is exclusively to prevent them from further participating in the armed conflict. It cannot and should not be construed as a criminal sanction. For further details, see Geneva Convention III Relative to the Treatment of Prisoners of War, 75 UNTS 135, 12 August 1949 (hereinafter, Geneva Convention III).

⁹⁰ See Articles on the Responsibility of States for Internationally Wrongful Acts, adopted by the International Law Commission on 10 August 2001, Report of the International Law Commission, Fifty-third Session, UN Doc. A/56/10, Arts. 4 and 8, respectively.

⁹¹ International law does not, itself, forbid members of non-international armed groups to fight. See text corresponding to note 104, infra.

⁹² AP I, Art. 1(4). The way in which the law has rendered this outcome is by claiming that conflicts of this particular type are ‘international armed conflicts’.

⁹³ This was one of the reasons the US declined ratifying Additional Protocol I. See, e.g., Letter of Transmittal from Reagan, Ronald, President of the United States, to United States Senate (20 January 1987), reprinted in (1987) 81 American Journal of International Law 910 CrossRefGoogle Scholar. This provision has also been included in relevant reservations by several countries, including France, the UK, Belgium, the Republic of Korea, Ireland, and Canada. On its lack of effective use, see D. Akande, ‘Classification of Armed Conflicts: Relevant Legal Concepts’, in supra note 88, at 49). Yet a rational reconstruction of the law certainly allows for considering this provision binding, at least as a matter of treaty law.

⁹⁴ Sandoz, Y. et al. (eds.), Commentary to the Additional Protocols of 9 June 1977 to the Geneva Conventions of 12 August 1949 (1987), 46 Google Scholar; Sloane, R., ‘The Cost of Conflation: Preserving the Dualism of Jus ad Bellum and Jus in Bello in the Contemporary Law of War’, (2009) 34 Yale Journal of International Law 47, at 65Google Scholar, and references therein. Other people invoke other grounds for this particular extension. Corn, e.g., suggests that the rationale for their different treatment was that these particular groups were generally not perceived as committing treason, and therefore did not clearly violate the sovereign right of states. Corn, G., ‘Thinking the Unthinkable: Has the Time Come to Offer Combatant Immunity to Non-State Actors?’, (2011) 22 (1) Stanford Law & Policy Review 253, at 281Google Scholar.

⁹⁵ See Y. Dinstein, The Conduct of Hostilities under the Law of International Armed Conflict (2004), 29; L. Oppenheim, International Law: Disputes, War and Neutrality (1952), 256; G. Schwarzenberger, International Law as Applied by International Courts and tribunals: The Law of Armed Conflict (1968), 115–17, among others.

⁹⁶ See, e.g., C. Garraway, ‘Interoperability and the Atlantic Divide – A Bridge over Troubled Waters’, 80 International Legal Materials 337, at 344; R. Baxter, ‘So-Called “Unprivileged Belligerency”: Spies, Guerrillas and Saboteurs’, 1951 British Yearbook of International Law 323–45.

⁹⁷ Ex parte Quirin, et al., 317 US 30, 31 (1942). See, similarly, the decision of the Judicial Committee of the Privy Council in Mohammed Ali v. Public Prosecutor, [1968] 3 All ER 488.

⁹⁸ Geneva Convention III. For a comprehensive list of the legal requirements, see Dinstein, supra note 95, at 37–40.

⁹⁹ Additional Protocol I, Art. 44(3). AP I not only allowed for combatants in certain non-state armed groups being considered privileged; it also controversially narrowed the category of unprivileged combatants. In short, it establishes that it sufficed for immunity from prosecution that they carry arms openly ‘within each individual engagement’, and they are ‘made visible to the adversary while engaged in a military deployment preceding the launching of an attack’ in which they would participate. Not surprisingly, this was heavily criticized by part of the international community, as well as by many prominent international law scholars. Yet, the main criticism of this extension was not that it was normatively flawed, but rather that it was ‘diluting one of the most important quid pro quos of humanitarian law’. Corn, supra note 94, at 274, and references in footnote 81.

¹⁰⁰ I am grateful to an anonymous reviewer for pressing me on this point.

¹⁰¹ It would be plausible to suggest that there may be further non-state armed groups which should be analogized to the three explicitly provided for here. Arguably, this would be the position most clearly compatible with a strong commitment with the respect and protection of fundamental human rights.

¹⁰² Any use of force that incurs in this type of behavior would be very unlikely to satisfy ad bellum necessity and proportionality. On these requirements, see, e.g., Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States), Merits, Judgment of 27 June 1986, [1996] ICJ Rep. 14, para. 176; Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion of 8 July 1996, [1996] ICJ Rep. 226, para. 41. For a more recent statement, see Tams, C., ‘Necessity and Proportionality’, in van den Herik, L. and Schrijver, N. (eds.), Counter-Terrorism Strategies in a Fragmented International Legal Order (2013)Google Scholar.

¹⁰³ For a possible exception, see supra note 63.

¹⁰⁴ Dinstein, supra note 95, at 234.

¹⁰⁵ Ibid., at 237.

¹⁰⁶ See ‘Siete detenidos por captar 12 mujeres para el Estado Islámico’, El País, 16 December 2014, available at politica.elpais.com/politica/2014/12/16/actualidad/1418717071_972920.html.

¹⁰⁷ On Belgium, see cat-int.org/wp-content/uploads/2017/04/Terrorist-attacks-Report-2013-2016.pdf.

¹⁰⁸ On England, see, e.g., www.independent.co.uk/news/uk/politics/plan-to-charge-jihadists-with-treason-will-not-work-claims-terror-expert-9804322.html, acknowledging liability but arguing against prosecution on policy grounds. For France, see www.trt.net.tr/francais/europe/2015/05/15/france-premiere-condamnation-d-un-combattant-a-l-etranger-276470; for Denmark, see www.thelocal.dk/20160622/denmark-convicts-first-isis-foreign-fighter; for a conviction for recruitment of foreign fighters in Australia, see www.scmp.com/news/asia/australasia/article/1988886/australian-convicted-recruiting-foreign-fighters-islamic-state. On Germany, see www.spiegel.de/international/germany/germany-faceschallenges-in-putting-islamic-state-radicals-on-trial-a-991744.html; for Italy, see, e.g., eblnews.com/news/europe/italy-court-issues-first-ever-foreign-fighter-terrorism-conviction-49029; for the Netherlands, see www.bbc.com/news/world-europe-35064597.

¹⁰⁹ Personal communication with Belgian prosecutor (on file with author).

¹¹⁰ Interestingly, the alleged charge against the UK nationals would be treason. www.independent.co.uk/news/world/middle-east/government-moots-treason-trials-for-returning-british-isis-fighters-9799968.html.

¹¹¹ In Belgium, for instance, the crime is often considered to be leaving the country with intent to participate in terrorist activity. See, e.g., decisions Tribunal de Première Instance de Bruxelles (70e), 10 March 2017; Cour d'Appel de Bruxelles (12e), 2 June 2017 (on file with author). For a critical analysis of this type of jurisdictional basis, see Chehtman, supra note 8, at 404.

¹¹² Although this article assumes that they would be largely provided within the laws on war crimes, this need not be the case. I would concede that this type of act can be subject to international criminalization on other grounds. This possibility, however, is beyond the scope of this article. I am grateful to an anonymous reviewer for pressing me on this matter.

¹¹³ ‘Nino – In His Own Words’, (2011) 22(4) European Journal of International Law 931, at 942 (Interview with Antonio Cassese).

¹¹⁴ At the very least, they would normally be free from ‘victor's justice’ and tu quoque accusations. On victor's justice, see, e.g., G. Bass, Stay the Hand of Vengeance: The Politics of War Crimes Trials (2000), Ch. 5; Simpson, supra note 76, Ch. 5. On tu quoque, see Yee, S., ‘The Tu Quoque Argument as a Defence to International Crimes, Prosecution, or Punishment’, (2004) 3 Chinese Journal of International Law 87 CrossRefGoogle Scholar.

¹¹⁵ K. Sikkink, The Justice Cascade. How Human Rights are Changing World Politics (2011).

¹¹⁶ See, inter alia, D. Luban, Legal Modernism (1997), 335–62; Alvarez, J., ‘Nuremberg Revisited: The Tadić Case’, in (1996) 7 European Journal of International Law 245 CrossRefGoogle Scholar.