CYIL vol. 9 (2018)

CYIL 9 ȍ2018Ȏ INTERNATIONAL LAW ON THE USE OF FORCE… we face an evolution or erosion of the norm, or even the process of its creation. 11 This is where practice becomes crucial with regard to hybrid, cyber threats, or issues concerning use of force in general, since the practice in this area challenges the current international legal system. However, assessment of practice raises several important questions: (1) What counts as State practice and what is the role of inaction? (2) Is practice also of other actors than States relevant? If so, is it necessary to distinguish according to the (norm-modifying, norm-generating, norm-interpreting) function of practice in the particular case? (3) Finally, are there any requirements, which the practice must fulfill? The abovementioned questions may be modified or adjusted depending on the specific character of the source of international law in question, i.e. depending on whether the practice is assessed against customary international law (as custom’s constituting aspect, i.e. being fundamental especially in the norm-creation process), or against conventional rules (for example in the form of subsequent practice of States). Since law of the use of force is of twofold character, customary and conventional, these two sources will constitute the focus of the following text. Importantly, it should be noted that in practice it is often difficult, if not impossible, to distinguish whether practice in a specific case is a proof of opinio iuris conventionalis or a confirmation of existing customary obligation ( opinio juris consuetidi or sive necessitatis ). 12 3.1 Verbal versus material acts As the ILC noted, practice may take a wide range of forms. 13 This includes both physical and verbal acts. Verbal acts are even more suitable examples of practice than physical acts, 14 because actors often express as a part of verbal acts their beliefs regarding what the law is. On the other hand, physical acts can only give an implicit indication of opinio juris – it cannot be assumed that implication of State’s physical act is a belief that the act is lawful. State action as such can be illegal, but the State can nevertheless attempt to justify the conduct (action or omission) with legal arguments. The justification has more legal significance than the conduct itself. Moreover, States may decide to act in violation of existing international legal norms deliberately. Reasons for such a conduct may be various, nevertheless it is for example widely accepted (despite the persisting legal murkiness of this area) that when replacing an old customary norm the States will necessarily engage in practice inconsistent with the 11 The ILC’s Commentary on Conclusion 4 (Requirement of practice) states that: “it is principally the practice of States that is to be looked to in determining the existence and content of rules of customary international law . ” ILC Report, A/71/10, 2016, chap. VI, paras. 64-76, p. 87. 12 In any search to determine whether [the concept of self-defense belongs] to customary or conventional international law it would appear to be a fallacy to try to split any concept to ascertain what part or percentage of it belongs to customary law and what fraction belongs to conventional law. Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America) . Merits, Judgment, I.C.J. Reports 1986, Separate opinion of Judge Singh, p. 152. 13 ILC Conclusion 6 (ILC Report, A/71/10, 2016, chap. V, paras. 50-63). Cf. also ILC’s Conclusion 6 para. 2 on Subsequent agreements and subsequent practice in relation to the interpretation of treaties (ILC Report, A/71/10, 2016, chap. VI, paras. 64-76). 14 O’CONNELL, M. E.: Taking Opinio Juris Seriously, A Classical Approach To International Law on the Use of Force. In: CANNIZZARO, E., PALCHETTI, P.: Customary International Law on the Use of Force: Methodological Approach . Brill: Leiden, 2005, p. 15. 3. What counts as relevant (State) practice?

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