1 By way of example, the conclusions of the European Council of 13–14 December 2012 state that ‘the European Council stresses that current financial constraints highlight the urgent necessity to strengthen European cooperation in order to develop military capabilities and fill the critical gaps, including those identified in recent operations.’ European Union, Conclusions of the European Council of 13–14 December 2012, 14 December 2012, EUCO 205.12, para. 22.

2 ICRC, International Humanitarian Law and the Challenges of Contemporary Armed Conflicts (hereinafter Challenges Report), October 2011, p. 32, available at: www.icrc.org/eng/assets/files/red-cross-crescent-movement/31st-international-conference/31-int-conference-ihl-challenges-report-11-5-1-2-en.pdf. All internet references were accessed in November 2013.

3 See for example Zwanenburg, Marten, ‘The duty to respect international humanitarian law during European Union-led operations’, in Sari, Aurel and Wessel, Ramses A. (eds.), Human Rights in EU Crisis Management Operations: A Duty to Respect and to Protect?, CLEER Working Papers 2012/6, 2012, pp. 63–78Google Scholar, available at: www.asser.nl/upload/documents/20121221T112600-CLEER%20Working%20Paper.pdf.

4 ICRC, Challenges Report, above note 2, p. 32. See also Marten Zwanenburg, ‘International organisations vs. troops contributing countries: which should be considered as the party to an armed conflict during peace operations?’, in International Organisations’ Involvement in Peace Operations: Applicable Legal Framework and the Issue of Responsibility, Proceedings of the 12th Bruges Colloquium, 20–21 October 2011, pp. 23–28, available at: www.coleurope.eu/sites/default/files/uploads/news/12th_bruges_colloquium_0.pdf.

5 See, for example, Garraway, Charles, ‘Interoperability and the Atlantic divide – a bridge over troubled waters’, in Israel Yearbook on Human Rights, Vol. 34, 2004, pp. 121–125Google Scholar.

6 ‘An operation conducted by forces of two or more nations acting together’: see NATO Standardization Agency (NSA), NATO Glossary of Terms and Definitions (English and French), AAP-06, 2013, p. 2-M-11, available at: http://nsa.nato.int/nsa/zPublic/ap/aap6/AAP-6.pdf.

7 This definition is adapted from the NATO definition of ‘force interoperability’: ‘The ability of the forces of two or more nations to train, exercise and operate effectively together in the execution of assigned missions and tasks.’ See NSA, above note 6, p. 2-F-5.

8 A JTAC is a qualified (certified) service member who, from a forward position, directs the action of combat aircraft engaged in close air support and other air operations. See United States Joint Chiefs of Staff, Joint Publication 3-09.3: Close Air Support, 8 July 2009, p. ix.

9 For a general discussion on the application of IHL to peace operations, see Greenwood, Christopher, ‘International humanitarian law and United Nations military operations’, in Yearbook of International Humanitarian Law, Vol. 1, 1998, pp. 3–34CrossRefGoogle Scholar.

10 The discussion in this section is based on Zwanenburg, Marten, ‘Legal interoperability in multinational forces: a military necessity’, in Proceedings of the Bruges Colloquium: Current Challenges to the Law of Occupation, 2005, pp. 108–115Google Scholar, available at: www.coleurope.eu/content/publications/pdf/Collegium%2034.pdf. For an academic discussion of the legal qualification of the ISAF operation, see Mortopoulos, Constantine D., ‘Could ISAF be a PSO? Theoretical extensions, practical problematic and the notion of neutrality’, in Journal of Conflict & Security Law, Vol. 15, 2010, pp. 573–587Google Scholar.

11 The Canadian government argued that IHL applied to detainees taken by Canadian forces in ISAF. See the report provided by Christopher Greenwood at the request of the Government for the case of Amnesty International Canada v. Canada (Chief of Defence Staff), 2008 FC 336, [2008] 4 FCR 540; Christopher Greenwood, Report: International Law Framework for the Treatment of Persons Detained in Afghanistan by Canadian Forces, p. 15, available at: http://bccla.org/wp-content/uploads/2012/04/Report-of-Prof.-Greenwood-London-School-of-Economics.pdf.

12 Kamerstukken II 2007–2008 (parliamentary papers), 27925, No. 287, p. 121.

13 Interview with Defence Minister zu Guttenberg, Bild, 3 November 2009, available at: www.bundesregierung.de/Content/DE/Interview/2009/11/2009-11-03-interview-guttenberg-bild.html.

14 Statement by Minister of Foreign Affairs Westerwelle in Parliament, 10 February 2010, available at: www.auswaertiges-amt.de/DE/Infoservice/Presse/Reden/2010/100210-BM-BT-Afghanistan.html.

15 For a discussion of this issue, see inter alia Lubell, Noam and Derejko, Nathan, ‘A global battlefield? Drones and the geographical scope of armed conflict’, in Journal of International Criminal Justice, Vol. 11, 2013, pp. 65–88CrossRefGoogle Scholar.

16 It may be noted that on occasion, the UN Security Council has referred to certain international obligations in the context of resolutions authorising the use of force by multinational military operations. Such references generally do not specify particular applicable treaties. For example, in Resolution 2011 extending the authorisation for ISAF, the Council called on ‘all parties to comply with their obligations under International Humanitarian Law’. UNSC Res. 2011, 12 October 2011, UN Doc. S/RES/2011, Preamble, para. 24.

17 The list of States Parties to the Additional Protocols is available at: www.icrc.org/applic/ihl/ihl.nsf/States.xsp?xp_viewStates=XPages_NORMStatesParties&xp_treatySelected=470 for Additional Protocol I (AP I), and www.icrc.org/applic/ihl/ihl.nsf/States.xsp?xp_viewStates=XPages_NORMStatesParties&xp_treatySelected=475 for Additional Protocol II (AP II).

18 For example, the 1954 Hague Convention on the Protection of Cultural Property in the Event of Armed Conflict and its two Protocols of 1954 and 1999, as well as the 2008 Convention on Cluster Munitions.

19 The ICJ held that actions by states ‘not only must amount to a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of the rule of law requiring it. The need for such a belief, i.e. the existence of a subjective element, is implicit in the very notion of the opinio iuris sive necessitatis. The States concerned must therefore feel that they are conforming to what amounts to a legal obligation.’ See North Sea Continental Shelf, Judgement, ICJ Reports 1969, p. 45, para. 77.

20 See d'Aspremont, Jean, ‘Théorie des sources’, in van Steenberghe, R. (ed.), Droit International Humanitaire: Un Régime Spécial de Droit International?, Bruylant, Brussels, 2013, p. 73Google Scholar.

21 Henckaerts, Jean-Marie and Doswald-Beck, Louise, Customary International Humanitarian Law, Cambridge University Press, Cambridge, 2006Google Scholar.

22 See generally Wilmshurst, Elizabeth (ed.), Perspectives on the ICRC Study on Customary International Humanitarian Law, Cambridge University Press, Cambridge, 2007Google Scholar; Bellinger, John and Haynes, William J., ‘A US government response to the International Committee of the Red Cross study on customary international humanitarian law’, in International Review of the Red Cross, Vol. 89, No. 866, June 2007, p. 433Google Scholar; and Newalsing, Eloisa, ‘Fruit of the loom: custom revisited’, in Leiden Journal of International Law, Vol. 21, 2008, p. 255Google Scholar.

23 This example assumes that the state does not accept Art. 52 of AP I as reflecting customary international law.

24 In the United States and NATO, this is referred to as the ‘nine-line briefing’. This refers to the nine subjects that the briefing covers. See US Joint Chiefs of Staff, above note 8, p. V-40.

25 Boddens Hosang, J. F. R., ‘Aandachtspunten in de ISAF ROE vanuit het Strategisch-juridisch Niveau’, in Militair Rechtelijk Tijdschrift, Vol. 105, No. 5, 2009, p. 219Google Scholar.

26 Kretzmer, David, ‘The Advisory Opinion: the light treatment of international humanitarian law’, in American Journal of International Law, Vol. 99, No. 1, 2005, pp. 88–102CrossRefGoogle Scholar.

27 On precedent in international courts, see generally Guillaume, Gilbert, ‘The use of precedent by international judges and arbitrators’, in International Journal of Dispute Settlement, Vol. 2, No. 1, 2011, pp. 5–23CrossRefGoogle Scholar.

28 See, for example, Operational Law Experts Roundtable on the Gotovina Judgement: Military Operations, Battlefield Reality and the Judgement's Impact on Effective Implementation and Enforcement of International Humanitarian Law, International Humanitarian Law Clinic, Emory Law School, 2012, available at: www.law.emory.edu/fileadmin/NEWWEBSITE/Centers_Clinics/IHLC/Gotovina_Meeting_Report.pdf.

29 Swart, Mia, ‘Judicial lawmaking at the ad hoc tribunals: the creative use of the sources of international law and “adventurous interpretation”’, in Zeitschrift für Ausländisches Öffentliches Recht und Völkerrecht, Vol. 70, 2010, p. 459Google Scholar. Swart criticises the definition of rape given by the ICTY Trial Chamber in the Furundžija case: see ibid., p. 485.

30 Solis, Gary D., The Law of Armed Conflict: International Humanitarian Law in War, Cambridge University Press, Cambridge, 2010, pp. 104–107CrossRefGoogle Scholar. The words ‘obsolete’ and ‘quaint’ were used by White House Counsel Roberto Gonzalez to describe certain provisions of the Geneva Conventions concerning treatment of detainees. See www2.gwu.edu/~nsarchiv/NSAEBB/NSAEBB127/02.01.25.pdf.

31 An example is the United Kingdom's attempts to foster an agreed interpretation of the IHL rules relevant to the protection of prisoners of war against public curiosity. The United Kingdom made a pledge to this effect at the 28^th International Conference of the Red Cross and Red Crescent: see www.icrc.org/Applic/p128e.nsf/pbk/DCOE-5TWNNH?OpenDocument&section=PBGO.

32 Memorandum for McNeill, John H., Assistant General Counsel (International), OSD (9 May 1986), in Law of War Documentary Supplement, United States Army Judge Advocate General's Legal Center and School, 2007, p. 399Google Scholar. Art. 52(3) provides: ‘In case of doubt whether an object which is normally dedicated to civilian purposes, such as a place of worship, a house or other dwelling or a school, is being used to make an effective contribution to military action, it shall be presumed not to be so used.’

33 Watkin, Kenneth, ‘Coalition operations: a Canadian perspective’, in Carsten, Michael D. (ed.), International Law and Military Operations, U.S. Naval War College International Law Studies, Vol. 84, 2008, p. 255Google Scholar.

34 Department of the Navy, The Commander's Handbook on the Law of Naval Operations, July 2007, pp. 5–2 and 5–3, available at: www.usnwc.edu/getattachment/a9b8e92d-2c8d-4779-9925-0defea93325c/.

35 Military Commissions Act of 2009, Pub. L. No. 111-84, para. 950(p), 123 Stat. 2190 (codified at 10 U.S.C. para. 47A (2006)).

36 United States Army Judge Advocate General's Legal Center and School, Operational Law Handbook, 2011, p. 132, available at: www.loc.gov/rr/frd/Military_Law/pdf/operational-law-handbook_2011.pdf.

37 In this context, ‘direct’ and ‘active’ are generally considered to be synonymous. See, for example, ICTR, Prosecutor v. Akayesu, Case No. ICTR-96-4-T, Judgement, 2 September 1998, para. 629.

38 Melzer, Nils, Interpretive Guidance on the Notion of Direct Participation in Hostilities under International Humanitarian Law, ICRC, Geneva, 2009Google Scholar (hereinafter ‘Interpretive Guidance’).

39 See, for example, Hays Parks, W., ‘Part IX of the ICRC “Direct Participation in Hostilities” study: no mandate, no expertise, and legally incorrect’, in New York University Journal of International Law and Politics, Vol. 42, 2010, pp. 768–830Google Scholar.

40 See Schmitt, Michael N., ‘Targeting narcoinsurgents in Afghanistan: the limits of international humanitarian law’, in Yearbook of International Humanitarian Law, Vol. 12, 2009, pp. 1–20CrossRefGoogle Scholar.

41 NATO, ‘NATO steps up counter-narcotics efforts in Afghanistan’, available at: www.nato.int/cps/en/natolive/news_50120.htm.

42 Susanne Koelbl, ‘Battling Afghan drug dealers: NATO high commander issues illegitimate order to kill’, in Der Spiegel Online, 28 January 2009, available at: www.spiegel.de/international/world/battling-afghan-drug-dealers-nato-high-commander-issues-illegitimate-order-to-kill-a-604183.html.

43 Ibid.

44 Pomper, Stephen, ‘Toward a limited consensus on the law of immunity in non-international armed conflict: making progress through practice’, in Watkin, Kenneth and Norris, Andrew J. (eds.), Non-International Armed Conflict in the Twenty-First Century, US Naval War College International Law Studies, Vol. 88, 2012, p. 182Google Scholar.

45 Keck, Trevor A., ‘Not all civilians are created equal: the principle of distinction, the question of direct participation in hostilities, and evolving restraints on the use of force in warfare’, in Military Law Review, Vol. 211, 2012, pp. 144–146Google Scholar.

46 James Risen, ‘U.S. to hunt down Afghan drug lords tied to Taliban’, in New York Times, 9 August 2009, available at: www.nytimes.com/2009/08/10/world/asia/10afghan.html?_r=0.

47 This observation is based on discussions by the author with legal advisers to governments of a number of other NATO Member States.

48 ILC, Draft Articles on Responsibility of States for Internationally Wrongful Acts, with commentaries, in Yearbook of the International Law Commission, 2001, Vol. 2, Part 2, available at: http://legal.un.org/ilc/texts/instruments/english/commentaries/9_6_2001.pdf.

49 Ibid., p. 36.

50 Ibid., p. 66.

51 See Boisson de Chazournes, Laurence and Condorelli, Luigi, ‘Common Article 1 of the Geneva Conventions revisited: protecting collective interests’, in International Review of the Red Cross, Vol. 82, 2000, pp. 67–87Google Scholar.

52 Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory, Advisory Opinion, ICJ Reports 2004, p. 136, para. 158.

53 Rome Statute of the International Criminal Court, 17 July 1998, 2187 UNTS 38544, Art. 25(3)(b).

54 Ibid., Art. 25(3)(c).

55 Ibid., Art. 25(3)(d).

56 Convention on Cluster Munitions, Dublin, 30 May 2008, reproduced in International Legal Materials, Vol. 48, 2009, p. 354.

57 See, for example, the Netherlands’ Explanatory Memorandum to the law on ratification of the Convention, Kamerstukken II (parliamentary papers), 2009–2010, 32187 (R 1902), No. 3, p. 8.

58 This is illustrated, inter alia, by a debate between the Dutch government and a number of parliamentarians, who understood the prohibition on assisting in the use of cluster munitions set out in the treaty as also including a prohibition on transit. See Kamerstukken II (parliamentary papers), 32187 (R1902) G, 21 January 2011; and Kamerstukken II, 32187 (R1902) K, 2 May 2011. See also Schwager, Elke, ‘The question of interoperability – interpretation of Articles 1 and 21 of the Convention on Cluster Munitions’, in Humanitäres Völkerrecht Informationsschriften, Vol. 21, No. 1, 2008, p. 247Google Scholar.

59 Catherine Wallis, ‘Legitimate targets of attack: considerations when targeting in a coalition’, in The Army Lawyer, December 2004, p. 51, available at: www.loc.gov/rr/frd/Military_Law/pdf/12-2004.pdf.

60 States that have made such a declaration include Canada, Germany, Italy, the Netherlands, and the United Kingdom.

61 McConachie, Vicki, ‘Coalition operations: a compromise or an accommodation’, in Carsten, Michael D., International Law and Military Operations, U.S. Naval War College International Law Studies, Vol. 84, 2008, p. 241Google Scholar.

62 Kamerstukken I (parliamentary papers), 2005–2006, 30300 X, A, p. 4.

63 US Department of Defence (DoD), Dir. 2311.01E, DoD Law of War Program, 9 May 2006, p. 2. The exact language is: ‘Ensure that the members of their DoD Components comply with the law of war during all armed conflicts, however such conflicts are characterized, and in all other military operations.’

64 See Bullman, Timothy P., ‘A dangerous guessing game disguised as an enlightened policy: United States laws of war obligations during military operations other than war’, in Military Law Review, Vol. 159, 1999, p. 174Google Scholar.

65 ISAF Commander, Tactical Directive, 30 December 2008, available at: www.nato.int/isaf/docu/official_texts/Tactical_Directive_090114.pdf.

66 ISAF Commander, Tactical Directive, 6 July 2009, available at: www.nato.int/isaf/docu/official_texts/Tactical_Directive_090706.pdf.

67 Ibid.

68 For a discussion on this issue, see M. Zwanenburg, above note 4. See also Engdahl, Ola, ‘Multinational peace operations forces involved in armed conflict: who are the parties?’, in Larsen, Kjetl Milutinovic, Guldahl, Camilla, and Nystuen, Gro (eds.), Searching for a ‘Principle of Humanity’ in International Humanitarian Law, Cambridge University Press, Cambridge, 2012, pp. 233–271CrossRefGoogle Scholar.

69 Within NATO, a caveat is defined as ‘any limitation, restriction, or constraint by a nation on its military forces or civilian elements under NATO command and control or otherwise available to NATO, that does not permit NATO commanders to deploy and employ these assets fully in line with the approved operation plan. Note: A caveat may apply inter alia to freedom of movement within the joint operations area and/or to compliance with the approved rules of engagement.’ See NSA, above note 6, p. 2-C-2.

70 As caveats are classified, it is not possible to cite them here. For an illustration of the use of caveats in practice, see Susanne Koelbl and Alexander Zsandar, ‘Not licensed to kill: German Special Forces in Afghanistan let Taliban commander escape’, in Der Spiegel Online, 19 May 2008, available at: www.spiegel.de/international/world/not-licensed-to-kill-german-special-forces-in-afghanistan-let-taliban-commander-escape-a-554033.html.

71 Geoffrey Corn, ‘Multinational operations, unity of effort, and the Law of Armed Conflict’, HPCR Working Paper series, 2009, pp. 25–26, available at: http://reliefweb.int/sites/reliefweb.int/files/resources/84B588812CF91A754925762E00194C41-Harvard_May2009.pdf.

72 Brown, Neil, ‘Issues arising from Coalition operations: an operational lawyer's perspective’, in Borch, Fred and Wilson, Paul (eds.), International Law and the War On Terror, U.S. Naval War College International Law Studies, Vol. 79, 2003, p. 231Google Scholar.

73 Stephens, Dale G., ‘Coalition warfare – challenges and opportunities’, in Israel Yearbook on Human Rights, Vol. 36, 2006, pp. 17Google Scholar and 21.

74 Ibid., p. 20.

75 ICRC, Challenges Report, above note 2, p. 33.