1 This has been empirically demonstrated in Morey, Daniel S., “Military Coalitions and the Outcome of Interstate Wars”, Foreign Policy Analysis, Vol. 12, No. 4, 2016Google Scholar.

2 On this see, Jolle Demmers and Lauren Gould, “The Remote Warfare Paradox: Democracies, Risk Aversion and Military Engagement”, in Alasdair Mckay, Abigail Watson and Megan Karlshøj-Pedersen (eds), Remote Warfare: Interdisciplinary Perspectives, E-International Relations, Bristol, 2021.

3 Cunningham, David E., “Blocking Resolution: How External States can Prolong Civil Wars”, Journal of Peace Research, Vol. 47, No. 2, 2010CrossRefGoogle Scholar.

4 The leading study on this topic is Aydin, Aysegul and Regan, Patrick M., “Networks of Third-Party Interveners and Civil War Duration”, European Journal of International Relations, Vol. 18, No. 3, 2012CrossRefGoogle Scholar.

5 On the proliferation of armed groups as an effect of external support, see Jackson, Jaime A., San-Akca, Belgin and Maoz, Zeev, “International Support Networks and the Calculus of Uprising”, Journal of Peace Research, Vol 57, No. 5, 2020CrossRefGoogle Scholar.

6 Moore, Matthew, “Selling to Both Sides: The Effects of Major Conventional Weapons Transfers on Civil War Severity and Duration”, International Interactions, Vol. 38, No. 3, 2012CrossRefGoogle Scholar, pp. 332 ff.

7 Giegerich, Bastian and Hlatky, Stéfanie von, “Experiences May Vary: NATO and Cultural Interoperability in Afghanistan”, Armed Forces & Society, Vol. 46, No. 3, 2020CrossRefGoogle Scholar.

8 Christopher G. Pernin et al., Targeted Interoperability: A New Imperative for Multinational Operations, RAND Corporation, Santa Monica, CA, 2019, pp. 109–118.

9 For an exhaustive account of humanitarian challenges posed by partnered warfare, see Cordula Droege and David Tuck, “Fighting Together: Obligations and Opportunities in Partnered Warfare”, Humanitarian Law and Policy Blog, 28 March 2017, available at: https://blogs.icrc.org/law-and-policy/2017/03/28/fighting-together-obligations-opportunities-partnered-warfare/ (all internet references were accessed in October 2021). See also Cordula Droege and David Tuck, “Fighting Together and IHL: Setting the Legal Framework”, Humanitarian Law and Policy Blog, 12 October 2017, available at: https://blogs.icrc.org/law-and-policy/2017/10/12/fighting-together-international-humanitarian-law-setting-legal-framework-1-2/; Cordula Droege and David Tuck, “Fighting Together and IHL: Ensuring Respect for the Law and Assessing Responsibility for Violations”, Humanitarian Law and Policy Blog, 17 October 2017, available at: https://blogs.icrc.org/law-and-policy/2017/10/17/fighting-together-international-humanitarian-law-ensuring-respect-law-assessing-responsibility-violations-2-2/.

10 See Finucane, Brian, “Partners and Legal Pitfalls”, International Law Studies, Vol. 92, 2016, pp. 414–424Google Scholar.

11 See ICRC, Allies, Partners and Proxies: Managing Support Relationships in Armed Conflict to Reduce the Human Cost of War, Geneva, March 2021.

12 Ibid., pp. 85–86 (on the legal and policy framework for internal readiness), 115–116 (on the role of judicial authorities in internal oversight).

13 See, for example, the two issues devoted to “military assistance on request” published by the Journal on the Use of Force and International Law, Vol. 7, Nos 1 and 2, 2020.

14 See Jens D. Ohlin (ed.), Research Handbook on Remote Warfare, Edward Elgar, Cheltenham, 2017.

15 The American Bar Association has published a number of articles and blog posts as part of its Proxy Warfare Project, available at: www.americanbar.org/groups/human_rights/reports/report-legal-framework-regulating-proxy-warfare-2019/.

16 Andreas Krieg and Jean-Marc Rickli, Surrogate Warfare: The Transformation of War in the Twenty-First Century, Georgetown University Press, Washington, DC, 2019.

17 ICRC, above note 11.

18 Financing is the lower end of the definition, being the most remote form of support. It is considered here because it can give rise to responsibility for aid or assistance if the conditions set out in Article 16 of the International Law Commission's (ILC) Draft Articles on Responsibility of States for Internationally Wrongful Acts are met. See ILC, “Draft Articles on Responsibility of States for Internationally Wrongful Acts, with Commentaries”, United Nations Yearbook of the International Law Commission, Vol. 2, No. 2, 2001 (ARSIWA), p. 66, and below in this section.

19 A support-based approach to determine when the contribution to multinational forces entails the application of IHL is proposed in Ferraro, Tristan, “The Applicability and Application of International Humanitarian Law to multinational Forces”, International Review of the Red Cross, Vol. 95, No. 891–892, 2013CrossRefGoogle Scholar.

20 ICRC, above note 11, p. 14.

21 Ibid., p. 18.

22 Helmut Philipp Aust, Complicity and the Law of State Responsibility, Cambridge University Press, Cambridge, 2011, Chap. 8.

23 A study of partnered operations in light of “international obligations connected to the conduct of others” was first proposed in Boutin, Berenice, “The Interplay of International Obligations Connected to the Conduct of Others: Toward a Framework of Mutual Compliance among States Engaged in Partnered Warfare”, International Law Studies, Vol. 96, 2020Google Scholar. For a comprehensive analysis of the legal framework of partnered operations, see Boutin, Berenice, “Responsibility in Connection with the Conduct of Military Partners”, Military Law and the Law of War Review, Vol. 56, No. 1, 2017Google Scholar.

24 Geneva Convention (I) for the Amelioration of the Condition of the Wounded and Sick in Armed Forces in the Field of 12 August 1949, 75 UNTS 31 (entered into force 21 October 1950), Art. 1; Jean-Marie Henckaerts and Louise Doswald-Beck (eds), Customary International Humanitarian Law, Vol. 1: Rules, Cambridge University Press, Cambridge, 2005, Rule 139, available at: https://ihl-databases.icrc.org/customary-ihl/eng/docs/v1.

25 ICRC, Commentary on the First Geneva Convention: Convention (I) for the Amelioration of the Condition of the Wounded and Sick in Armed Forces in the Field, 2nd ed., Geneva, 2016, para. 167.

26 Ibid., para. 158. According to the ICRC Commentary, this obligation can require States to opt out of a multinational operation if there is an expectation that it would violate the Geneva Conventions: ibid., para. 161.

27 Ibid., para. 164.

28 Ibid., para. 125. A recent study on the obligation to “ensure respect” is Eve Massingham and Annabel McConnachie, Ensuring Respect for International Humanitarian Law, Routledge, London, 2021.

29 Droege, Cordula, “Transfers of Detainees: Legal Framework, Non-Refoulement and Contemporary Challenges”, International Review of the Red Cross, Vol. 90, No. 669, 2008, p. 683CrossRefGoogle Scholar.

30 Human Rights Committee, General Comment No. 36, “Article 6 (Right to Life)”, UN Doc. CCPR/C/GC/36, 3 September 2019, para. 63.

31 Arms Trade Treaty, 3013 UNTS, 2 April 2013 (entered into force 24 December 2014), Arts 6(3), 7.

32 See UNGA Res. 3314 (XXIX), “Definition of Aggression”, 14 December 1974, Annex, Art. 3(f–g).

33 To confirm the topicality of the subject, three monographs have been published recently on complicity in international law. See Vladyslav Lanovoy, Complicity and Its Limits in the Law of International Responsibility, Hart, Oxford and Portland, OR, 2016; Miles Jackson, Complicity in International Law, Oxford University Press, Oxford, 2015; H. P. Aust, above note 22.

34 ARSIWA, above note 18.

35 A factor further limiting the outcome of the investigation was of course the accessibility of sources in a language known to the author.

36 Foreign Assistance Act, US Public Law 87-195, Section 620M (US Code, Title 22, Section 2378d). Before 2008, the amendment had been included in all annual Foreign Operations Appropriations Acts since 1998.

37 US Code, Title 10, Section 362.

38 US Code, Title 22, Section 2378d(c).

39 Foreign Assistance Act, above note 36, Section 502B; however, and crucially, the US Government never enforced this provision.

40 See US Department of State, “About the Leahy Law”, 20 January 2021, pp. 12–13, available at: www.state.gov/key-topics-bureau-of-democracy-human-rights-and-labor/human-rights/leahy-law-fact-sheet/.

41 Ibid., p. 20.

42 See Daniel R. Mahanty, “The ‘Leahy Law’ Prohibiting US Assistance to Human Rights Abusers: Pulling Back the Curtain”, Just Security, 27 June 2017, available at: www.justsecurity.org/42578/leahy-law-prohibiting-assistance-human-rights-abusers-pulling-curtain/.

43 See the subsection on “State Policies Guiding the Conduct of National Institutions in Partnered Operations” below.

44 Between 2011 and 2013, the US Government subjected 530,000 foreign units to Leahy vetting. See Congressional Record, Vol. 160, No. 109 (Senate), 14 July 2014, pp. S4452–S4453.

45 Nina M. Serafino et al., “Leahy Law” Human Rights Provisions and Security Assistance: Issue Overview, Congressional Research Service Report R43361, 29 January 2014.

46 Miller, Nathanael T., “Leahy Law: Congressional Failure, Executive Overreach, and the Consequences”, Cornell International Law Journal, Vol. 45, No. 3, 2012Google Scholar.

47 D. R. Mahanty, above note 42.

48 US Department of State, “Introduction to Leahy Vetting Policy, Version 2.1”, available at: www.state.gov/wp-content/uploads/2020/06/PP410_INVEST_v2.1.pdf.

49 National Defense Authorization Act for Fiscal Year 2015, US Public Law 113-291, Section 1209.

50 Sarah Margon, “Is Credible Vetting of the Syrian Opposition for Human Rights Abuses Possible?”, Just Security, 24 September 2014, available at: www.justsecurity.org/15506/guest-post-credible-vetting-syrian-opposition-humanitarian-abuses-possible/.

51 Erica L. Gaston, Regulating Irregular Actors: Can Due Diligence Checks Mitigate the Risks of Working with Non-State and Substate Forces?, Working Paper 608, Centre for the Study of Armed Groups, Overseas Development Institute, London, May 2021.

52 “The term ‘appropriately vetted’ means … at a minimum: (A) assessments of such elements, groups, and individuals for associations with terrorist groups …; and (B) a commitment from such elements, groups, and individuals to promoting the respect for human rights and the rule of law.” US Public Law 113-291, Section 1209.

53 Law No. 145, 21 July 2016, Art. 1.

54 Ibid., Art. 2(2).

55 Ibid., Art.3(1).

56 Ibid., Art. 1.

57 Decree-Law No. 174, 30 October 2015, Converted with Modifications from Law No. 198, 11 December 2015, Art. 7bis. On this issue, see Natalino Ronzitti, “La legge italiana sulle missioni internazionali”, Rivista di Diritto Internazionale, Vol. 100, No. 2, 2017.

58 Luca Buscema, “Le operazioni umanitarie e di peacekeeping ed il valore costituzionale della pace alla luce della L. N. 145/2016”, Rivista AIC, No. 2/2018, 2018, pp. 17–21.

59 Claire Mills, Parliamentary Approval for Military Action, House of Commons Library Briefing Paper CBP 7166, 8 May 2018, p. 36.

60 Ibid.

61 Emily Knowles and Abigail Watson, Lawful but Awful? Legal and Political Challenges of Remote Warfare and Working with Partners, Oxford Research Group, May 2018, p. 18.

62 James Strong, “The War Powers of the British Parliament: What Has Been Established and What Remains Unclear?”, British Journal of Politics and International Relations, Vol. 20, No. 1, 2018, p. 27.

63 Secretary of State for Defence Michael Fallon, Written Statement, UIN HCWS678, 18 April 2016.

64 N. Ronzitti, above note 57, p. 478.

65 German Federal Administrative Court, Attorney of the Federal Armed Forces v. N (a Major of the Federal Armed Forces), Case No. 2 WD 12.04, Judgment, 21 June 2005.

66 Ibid., para. 4.1.4.1.4.

67 UNGA Res. 3314 (XXIX), above note 32.

68 German Federal Administrative Court, N, above note 65, para. 4.1.4.1.4.

69 V. Lanovoy, above note 33, p. 183.

70 The claimants were supported by the European Center for Constitutional and Human Rights (ECCHR). For additional information on the case, see ECCHR, “Ramstein at Court: Germany's Role in US Drone Strikes in Yemen”, available at: www.ecchr.eu/en/case/important-judgment-germany-obliged-to-scrutinize-us-drone-strikes-via-ramstein/.

71 Administrative Court of Cologne, Jaber v. Federal Government of Germany, Case No. 3 K 5625/14, Judgment, 27 May 2015.

72 Higher Administrative Court for North Rhine-Westphalia, Jaber v. Federal Government of Germany, Case No. 4 A 1361/15, Judgment, 19 March 2019.

73 As recognized by the Basic Law (Grundgesetz), Art. 2(2).

74 Higher Administrative Court for North Rhine-Westphalia, Jaber, above note 72, para. III.2.a.

75 Ibid., incipit.

76 Ibid., para. III.1.a.aa.2.

77 German Federal Administrative Court, Jaber v. Federal Government of Germany, Case No. 6 C 7.19, Judgment, 25 November 2020. The plaintiff submitted a constitutional complaint, which is currently pending before the Federal Constitutional Court.

78 For a comment on this decision, see Karoline Dołgowski, “One Step Forward – Three Steps Back: Why the Hellfire from Ramstein may Continue After All”, Voelkerrechtsblog, 1 December 2020, available at: https://voelkerrechtsblog.org/one-step-forward-three-steps-back-why-the-hellfire-from-ramstein-may-continue-after-all/.

79 High Court of Eastern Denmark, X v. Ministry of Defence, Case No. B-3448-14, Judgment, 15 June 2018. The appeal is currently pending before the Danish Supreme Court.

80 Ibid., pp. 810–811.

81 This stance brought the Danish MoD to immediately voice its disappointment. See Thomas Obel Hansen and Fiona Nelson, “Liability of an Assisting Army for Detainee Abuse by Local Forces: The Danish High Court Judgment in Green Desert”, EJIL: Talk!, 24 January 2019, available at: www.ejiltalk.org/liability-of-an-assisting-army-for-detainee-abuse-by-local-forces-the-danish-high-court-judgment-in-green-desert/.

82 Peter Vedel Kessing, “Liability in Joint Military Operations: The Green Desert Case”, Journal of Conflict and Security Law, Vol. 25, No. 2, 2020.

83 Court of Appeal of England and Wales (Civil Division), R (Noor Khan) v. Secretary of State for Foreign and Commonwealth Affairs, Citation No. [2014] EWCA Civ 24, Judgment, 20 January 2014.

84 Ibid., para. 8.

85 UK Supreme Court, Belhaj and Another v. Straw and Others; Rahmatullah (No. 1) v. Ministry of Defence, Citation No. [2017] UKSC 3, Judgment, 17 January 2017.

86 Ibid., paras 4, 6.

87 Ibid., para. 168.

88 Ibid., para. 93.

89 Shaheed Fatima, “Noor Khan: A Missed Opportunity?”, Just Security, 30 January 2014, available at: www.justsecurity.org/6530/noor-khan-missed-opportunity/.

90 A relevant example is the ICRC's updated IHL implementation guidance, which takes into account “administrative and practical measures”, including the creation of relevant institutions, processes and procedures. see ICRC, Bringing IHL Home: Guidelines on the National Implementation of International Humanitarian Law, Geneva, May 2021, pp. 15–24.

91 HM Government, Overseas Security and Justice Assistance Guidance, London, 26 January 2019 (OSJA Guidance).

92 Hannah Wright, Overseas Security and Justice Assistance: An Assessment of Human Rights Guidance, Saferworld, London, February 2012; Maya Foa, “The British Government is Covering Up Its Assistance to Torturers and Killers Worldwide”, OpenDemocracy, 25 November 2016, available at: www.opendemocracy.net/en/openjustice/uk-remains-complicit-in-horrific-human-rights-abuses-abroad-due-to-shoddy-policy-document/.

93 HM Government, The Principles relating to the Detention and Interviewing of Detainees Overseas and the Passing and Receipt of Intelligence relating to Detainees (the Principles), London, 24 September 2019.

94 BBC, “UK Urged to Reveal ‘Torture’ File”, 28 July 2009, available at: http://news.bbc.co.uk/2/hi/uk_news/8170594.stm.

95 OSJA Guidance, above note 91, p. 4.

96 The Principles, above note 93, pp. 3–4.

97 This is the aim of the “Assessment and Approvals Process” under the OSJA Guidance, above note 91, p. 9.

98 The Principles, above note 93, p. 4.

99 Ruth Blakeley and Sam Raphael, “Accountability, Denial and the Future-Proofing of British Torture”, International Affairs, Vol. 96, No. 3, 2020, pp. 704–707.

100 Information Commissioner's Office, Decision No. FS50599866, 26 May 2016.

101 All Party Parliamentary Group on Drones, Exchange of Letters between Tom Watson MP and David Davis MP, and the Foreign and Commonwealth Office, 19 November 2014.

102 See the subsection on “Domestic Legislation Providing Conditions for Participation in Partnered Operations” above.

103 US President, Report on the Legal and Policy Frameworks Guiding the United States’ Use of Military Force and Related National Security Operations, Washington, DC, December 2016.

104 Ibid., p. 12 (emphasis added).

105 See Egan, Brian, “International Law, Legal Diplomacy, and the Counter-ISIL Campaign: Some Observations”, International Law Studies, Vol. 92, 2016 p. 245Google Scholar. As explained by Egan, the United States instead denies that common Article 1 legally requires States to take steps to ensure respect for IHL by all State and non-State parties to a conflict, including non-partners.

106 US Policy on Pre- and Post-Strike Measures to Address Civilian Casualties in US Operations Involving the Use of Force, Executive Order 13732, 1 July 2016, Section 2(b)(iii).

107 See, for example, DoD, Annual Report on Civilian Casualties in Connection with United States Military Operations in 2020, April 2021.

108 Elision and secrecy cover not only the existence of partnered operations, but also the impact of implementation measures where they exist. For a call to more transparency on the impact of the Leahy Law for example, see Michael J. McNerney et al., Improving Implementation of the Department of Defense Leahy Law, RAND Corporation, Santa Monica, CA, 2017, pp. 52–54.

109 ICRC, above note 11, Chaps 5, 6.

110 NATO doctrine sets out the three stages of “preparation”, “execution” and “operations assessment”. See NATO, Allied Joint Doctrine for the Conduct of Operations, NATO Standard AJP-3, February 2019. The US Army's operations process consists of “planning”, “preparation” and “execution”, with “assessment” being continuously ongoing. See US Army, The Operations Process, Army Doctrine Publication No. 5-0, 31 July 2019.

111 For both these interpretive issues, see the discussion on the Leahy Law in the subsection on “Domestic Legislation Providing Conditions for Participation in Partnered Operations” above.

112 ICRC, above note 11, p. 115.

113 See, for example, the freedom of information litigation filed by ECCHR over Italy's involvement in the US drone programme, available at: www.ecchr.eu/en/case/sicily-air-base-freedom-of-information-litigation-on-italys-involvement-in-us-drone-program/?L=2.

114 B. Egan, above note 105, p. 236.