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Published online by Cambridge University Press: 07 July 2009
On 27 June 1986 the International Court of Justice passed judgment in the case concerning military and paramilitary activities in and against Nicaragua. Because of a reservation that the United States had made when it accepted the jurisdiction of the Court, the Court could not pronounce a decision regarding the dispute insofar as it concerned multilateral convertions. As a resuld of this, the Court was compelled to base its judgment largely on rules of customary international law and general principles of law. The rules of customary law which were relevant for the judgment corresponded to a significant extent, as regards their content, to the rules of treaty law which the Court was unable to apply, such as the prohibition on the use of force of Article 2, paragraph 4 of the Charter of the United Nations. This led the Court to indicate in precise terms how rules of treaty law and rules of customary law which have a corressponding content can co-exist and how the existence of rules of customary international law can be established in general. In addition, the Court examined in some detail the existence and content of certain specific rules of customary international law.
1. Case concerning Military and Paramilitary Activities in and against Nicaragua, Merits, Judgment; ICJ Rep. 1986 p. 14. This case will be referred to below as the Nicaragua case.
2. This reservation on multilateral conventions, which was part of the declaration in pursuance of Art. 36, para. 2 of the Statute of the Court, by which the jurisdiction of the Court was recognized, excludes from this jurisdiction ‘disputes arising under a multilateral treaty, unless (1) all parties to the treaty affected by the decision are also parties to the case before the Court, or (2) the United States of America specially agrees to jurisdiction’.
3. Para. 183.
4. Para. 185.
5. Ibid.
6. Para. 186: ‘In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules, and that instances of State conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule. If a State acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State's conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule’.
7. ICJ Rep. 1969 para 77. This judgment will be referred to hereafter as the NSCS cases.
8. ‘Or other States in a position to react to it’. This formulation seems to express not only that it must be possible for States to respond, but also that it should be logical that they would do so.
9. This point is supplementary to the formulation used in the NSCS cases. We are concerned here with a response to an act which is in conformity with a settled practice. This means that the response must exist in the absence of protest if it is to reveal the view sought.
10. The passage concerned in para. 207 reads as follows: ‘… for a new customary rule to be formed, not only must the acts concerned “amount to a settled practice”, but they must be accompanied by the opinio juris sive necessitate. Either the States taking such action or other States in a position to react to it, must have behaved so that their conduct is “evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it”.’
11. The case of the S.S. ‘Lotus’, PCIJ 1927, ser A. No. 10.
12. Ibid.
13. ICJ Rep. 1984 p. 424, para. 73.
14. Fisheries case, p. 133.
15. ICJ Rep. 1949 p. 35.
16. Para. 202.
17. See para. 202 and above, section 3.1.
18. Para. 205. In this regard the Court states that ‘[a] prohibited intervention must accordingly be one bearing on matters in which each State is permitted, by the principle of State sovereignty, to decide freely. One of these is the choice of a political, economic, social and cultural system, and the formulation of foreign policy. Intervention is wrongful when it uses methods of coercion in regard to such choices, which must remain free ones’.
19. Emphasis added.
20. Para. 77.
21. Para. 207.
22. In para. 207 of the Nicaragua case, too, the Court examined whether an existing practice was the expression of a rule of customary international law.
23. Para. 188. Other places where the Court refers to such recognition as ‘opinio juris’ are paras. 184, 189, 191 and 202.
24. Para. 185.
25. Para. 188.
26. Para. 191 (emphasis added).
27. Para. 188.
28. The jurisdiction was also based on a compromissory clause in the 1959 Treaty of Friendship, Commerce and Navigation. This is of no further relevance for the discussion of customary international law.
29. This includes hereafter declarations under Art. 36(5).
30. Art. 36(2).
31. Para. 207.
32. Art. 59 of the Statute.
33. Para. 29.
34. R.Y. Jennings, in the words of SirBlackstone, William, ‘What is International Law, and How do we Tell It when we See It?’, 37 ASDI (1981) p. 74.Google Scholar
35. Emphasis added.
36. ICJ Rep. 1974 p. 268, para. 46; p. 473, para. 49.
37. Nuclear Tests cases, para. 47.
38. Para. 27.
39. See para. 43 of the judgment in the Nuclear Tests cases.
40. Nuclear Tests cases, para. 51.
41. ‘Synallagmatic’ means ‘pertaining to or of the nature of a contract or mutual engagement; imposing mutual obligations, reciprocally binding’ (Shorter Oxford Dictionary, 3rd edn.).
42. Para. 60 of the decision on jurisdiction.
43. Para. 45, Nuclear Tests cases.
44. ICJ Rep. 1961 p. 31. Quoted in para. 45 of the judgment in the Nuclear Tests cases.
45. See paras. 29 and 184.
46. See para. 185.
47. See para. 186.
48. Para. 184.
49. Para. 184. Emphasis added.
50. See, for example, Schreuer, C., ‘Recommendations and the Thiditional Sources of International Law’, 20 GYIL (1977) p. 103Google Scholar, and Jennings, R.Y., ‘The Identification of International Law’, in Bin, Cheng, ed., International Law: Teaching and Practice (1982) pp. 3–9.Google Scholar
51. Nuclear Tests cases, para. 46.
52. Bin Cheng, op. cit. n. 50.
53. Ibid. p. 6.
54. Ibid. p. 9.
