1 This contribution is in part based on Lijnzaad, Liesbeth, “The San Remo Manual on the Law of Naval Warfare – From Restatement to Development?”, in Klein, Natalie (ed.), Unconventional Lawmaking in the Law of the Sea, Oxford University Press, Oxford, 2022Google Scholar.

2 Aust, Anthony, “The Theory and Practice of Informal International Instruments”, International & Comparative Law Quarterly, Vol. 35, 1986CrossRefGoogle Scholar.

3 Pauwelyn, Joost, “Informal International Lawmaking: Framing the Concept and Research Questions”, in Pauwelyn, Joost, Wessel, Ramses A. and Wouters, Jan (eds), Informal International Lawmaking, Oxford University Press, Oxford, 2012, pp. 15–22CrossRefGoogle Scholar. See also Natalie Klein, “Meaning, Scope and Significance of Informal Lawmaking in the Law of the Sea”, in N. Klein (ed.), above note 1, pp. 6–13.

4 All the informal documents listed here drafted by groups of experts bear the title of “Manual” (with the name of the city where discussions took place added), as opposed to more varied titles for ICRC documents.

5 Doswald-Beck, Louise (ed.), San Remo Manual on International Law Applicable to Armed Conflicts at Sea, Cambridge University Press, Cambridge, 1995 (1994 San Remo Manual)CrossRefGoogle Scholar.

6 Developed under the auspices of the San Remo International Institute of Humanitarian Law: M. N. Schmitt, C. H. B. Garraway and Y. Dinstein, The Manual on the Law of Non-International Armed Conflict, International Institute of Humanitarian Law, Sanremo, 2006.

7 Program on Humanitarian Policy and Conflict Research at Harvard University, HCPR Manual on International Law Applicable to Air and Missile Warfare, Bern, 15 May 2009, available at: https://reliefweb.int/report/world/manual-international-law-applicable-air-and-missile-warfare (all internet references were accessed in September 2022).

8 Schmitt, Michael N. (ed.), Tallinn Manual on the International Law Applicable to Cyber Warfare, Cambridge University Press, Cambridge, 2013CrossRefGoogle Scholar.

9 Michael N. Schmitt (ed.), Tallinn Manual 2.0 on the International Law Applicable to Cyber Operations, 2nd ed., Cambridge University Press, Cambridge, 2017. Technically this does not deal with the law of armed conflict but with attacks below the threshold of armed conflict.

10 Gill, Terry D., Fleck, Dieter, Boothby, William H. and Vanheusden, Alfons (eds), Leuven Manual on the International Law Applicable to Peace Operations, Cambridge University Press, Cambridge, 2017CrossRefGoogle Scholar.

11 Dinstein, Yoram and Dahl, Arne Willy (eds), Oslo Manual on Select Topics of the Law of Armed Conflict, Springer Open, New York, 2020CrossRefGoogle Scholar.

12 Adelaide Law School, Australia, Woomera Manual, available at: https://law.adelaide.edu.au/woomera/.

13 Jean-Marie Henckaerts and Louise Doswald-Beck (eds), Customary International Humanitarian Law, Vol. 1: Rules, Cambridge University Press, Cambridge, 2005 (ICRC Customary Law Study), available at: https://ihl-databases.icrc.org/customary-ihl/eng/docs/v1. This work was based on a decision by the 1996 International Conference of the Red Cross and Red Crescent Movement.

14 Nils Melzer, Interpretative Guidance on the Notion of Direct Participation in Hostilities under International Humanitarian Law, ICRC, Geneva, 2009.

15 ICRC, “Expert Meeting: Occupation and Other Forms of Administration of Foreign Territory”, 11 June 2020, available at: www.icrc.org/en/publication/4094-occupation-and-other-forms-administration-foreign-territory-expert-meeting.

16 ICRC, “Guidelines on the Protection of Natural Environment in Armed Conflict”, available at: www.icrc.org/en/document/guidelines-protection-natural-environment-armed-conflict-rules-and-recommendations-relating. The International Law Commission (ILC) is working on a somewhat related project: protection of the environment in relation to armed conflicts; see ILC, Analytical Guide to the Work of the International Law Commission: Protection of the Environment in Relation to Armed Conflicts, General Assembly Action, Resolution 75/135 of 15 December 2020, available at: https://legal.un.org/ilc/guide/8_7.shtml#fout.

17 Swiss Federal Department of Foreign Affairs and ICRC, The Montreux Document on Pertinent International Legal Obligations and Good Practices for States Related to Operations of Private Military and Security Companies During Armed Conflict, Montreux, 17 September 2008, available at: www.eda.admin.ch/eda/en/fdfa/foreign-policy/international-law/international-humanitarian-law/private-military-security-companies/montreux-document.html.

18 On these two distinct types of military manuals, see Earle A. Partington, “Manuals on the Law of Armed Conflict”, in Max Planck Encyclopedias of International Law, August 2016, available at: https://opil.ouplaw.com/view/10.1093/law:epil/9780199231690/law-9780199231690-e326?rskey=QJAxQq&result=1&prd=MPIL. About relying on an informal manual in order to formulate a formal manual, see Steven Haines, “The United Kingdom's Manual of the Law of Armed Conflict and the San Remo Manual: Maritime Rules Compared”, in Yoram Dinstein and Fania Domb (eds), Israel Yearbook on Human Rights, Vol. 36, Martinus Nijhoff Publishers, Leiden/Boston, MA, 2006.

19 This distinction is a traditional one. With the 1977 Additional Protocols to the 1949 Geneva Conventions the distinction has to a certain extent disappeared as many roles on the methods and means of warfare have now been included in the Additional Protocols.

20 Hague Convention (VI) on Enemy Merchant Ships, Hague Convention (VII) on Conversion of Merchant Ships, Hague Convention (VIII) on Submarine Mines, Hague Convention (IX) on Bombardment by Naval Forces, Hague Convention (XI) on Restrictions of the Right of Capture, Hague Convention (XIII) on Neutral Powers in Naval War, all 18 October 1907.

21 Apart from the adoption of Geneva Convention II in 1949 and some provisions in AP I and AP II in 1977.

22 Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques, New York, 10 December 1976, 1108 UNTS 151 (entered into force 5 October 1978).

23 There is some evidence that States have been aware of this issue, but have not persevered in taking it up. See Roach, J. Ashley, “The Law of Naval Warfare at the Turn of Two Centuries”, American Journal of International Law, Vol. 94, No. 1, 2000, p. 77CrossRefGoogle Scholar.

24 The matter of the (retroactive) applicability of the VCLT will not be elaborated upon here.

25 There are distinct rules with respect to revising Annex I of AP I in Article 98 of AP I which will not be discussed here. See Bruno Zimmermann, “Article 97 – Amendment”, in Yves Sandoz, Christophe Swinarski and Bruno Zimmermann (eds), Commentary on the Additional Protocols of 8 June 1977 to the Geneva Conventions of 12 August 1949, ICRC, Geneva, 1987, pp. 1093–7.

26 Such as the 1994 Agreement relating to the Implementation of Part XI of the United Nations Convention on the Law of the Sea of 10 December 1982; or the 1995 Agreement for the Implementation of the Provisions of the United Nations Convention on the Law of the Sea of 10 December 1982 relating to the Conservation and Management of Straddling Fish Stocks and Highly Migratory Fish Stocks.

27 Petrov, Anton Orlinov, Expert Laws of War: Restating and Making Law in Expert Processes, Edward Elgar Publishing, Cheltenham, 2020, pp. 32–7CrossRefGoogle Scholar.

28 J. Pauwelyn, above note 3.

29 N. Klein, above note 3.

30 Michael Bothe, “Private Normunternehmer im Völkerrecht: Gedanken zur Fortentwicklung des Völkerrechts durch nicht-staatliche Institutionen”, in Holger H. Hestermeyer et al. (eds), Coexistence, Cooperation and Solidarity: Liber Amicorum Rüdiger Wolfrum, Vol. II, Martinus Nijhoff Publishers, Leiden and Boston, MA, 2012.

31 See, for example, John B. Bellinger III and William J. Haines II, “A US Government Response to the International Committee of the Red Cross Study Customary International Humanitarian Law”, International Review of the Red Cross, Vol. 89, No. 866, 2007; or Jane Dalton, “A Comparison between the San Remo Manual and the U.S. Navy's Commander Handbook”, in Y. Dinstein and F. Domb (eds), above note 18, criticizing a number of rules of the San Remo Manual. Similarly, there has been criticism of the ICRC's Interpretative Guidance (N. Melzer, above note 14), for example: Parks, W. Hays, “Part IX of the ‘ICRC Direct Participation in Hostilities’ Study: No Mandate, No Expertise, and Legally Incorrect”, New York University Journal of International Law and Politics, Vol. 42, No. 3, 2010Google Scholar; and Boothby, William, “Direct Participation in Hostilies – A Discussion of the ICRC Interpretative Guidance”, Journal of International Humanitarian Legal Studies, Vol. 1, No. 1, 2010CrossRefGoogle Scholar.

32 This begs the question whether the idea of these being independent legal experts can be maintained. Looking back at the process that established the 1994 San Remo Manual, L. Doswald-Beck, above note 5, at p. 67, mentions that “Overall, about a third of the participants were academic personnel and the others were governmental personnel attending in their personal capacity”. Not only do these groups work on the basis of Chatham House rules on confidentiality, but the experts concerned also make a point of stressing that their views should not be understood as their (former) employers’ views. The composition of these groups is not always transparent, particularly when discussions end in discontent and a lack of consensus. Petrov, in discussing the fate of the ICRC's Interpretative Guidance (N. Melzer, above note 14), refers to this as “… Mainly Unrevealed Experts”. A. O. Petrov, above note 27, pp. 46–8. Other publications give lists of participants and their affiliations; see, for example, L. Doswald-Beck (ed.), above note 5, pp. 47–55; or M. N. Schmitt (ed.), above note 8, pp. x–xiii.

33 See, for example, mentioning support from governments, academia and the Red Cross movement: L. Doswald-Beck (ed.), above note 5, pp. 64–6.

34 Dapo Akande and Marko Milanovic, “International Law and Contemporary Security Challenges”, YouTube, 2 February 2021, at: www.youtube.com/watch?v=lTTz4Cc7mpU&feature=youtu.be. The discussion has now unfortunately been taken down.

35 This is not the place to discuss the composition of these groups of independent experts in detail. However, a number of people seem to participate in many (if not almost all) of these groups. While this is a testament to their impressive expertise, it also suggests a lack of inclusiveness as the development of these instruments thus seems to lie in the hands of a limited number of (mostly Western) men. Observing that the outcome of an expert process depends mainly on the individual group members, A. O. Petrov, above note 27, p. 79 at footnote 397, lists four experts who have participated in many processes. This list is perhaps not complete, and frequent participation does not necessarily imply having an impact during group discussions.

36 Such as reliance on the 1994 San Remo Manual by Germany, the United Kingdom and the United States; see Marco Sassòli, Antoine A. Bouvier and Anne Quintin, How Does Law Protect in War, Vol. I, 3rd ed., ICRC, Geneva, 2011, footnote 289, available at: https://www.icrc.org/en/doc/assets/files/publications/icrc-0739-part-i.pdf.

37 See above note 31.

38 On the persistent objector, see Olufemi Elias, “Persistent Objector”, in Max Planck Encyclopedias of International Law, September 2006, available at: https://opil.ouplaw.com/view/10.1093/law:epil/9780199231690/law-9780199231690-e1455?rskey=DAqgHS&result=1&prd=MPIL.

39 See the plans to update the 1994 San Remo Manual (started in 2019). The Oslo Manual is an update of the 2009 Harvard Manual on International Law Applicable to Air and Missile Warfare. Also there is the current updating process of the Tallinn Manual 2.0 on Cyber Operations. See, for example, an invitation by the North Atlantic Treaty Organization (NATO) Cooperative Cyber Defence Centre of Excellence (CCDCOE) calling for comments on Tallinn Manual 2.0 in preparation of the Tallinn Manual 3.0, asking for “experts to share their comments and suggestions on how the rules and accompanying commentary of the Tallinn Manual 2.0 should be revised in the light of emerging State practice”. CCDCOE, “The CCDCOE Invites Experts to Contribute to the Tallinn Manual 3.0”, available at: https://ccdcoe.org/news/2021/the-ccdcoe-invites-experts-to-contribute-to-the-tallinn-manual-3-0/.