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In addition, the persistent objection of States may be relevant after the formation of thatrule by preventing the application of that rule to States that have objected to that rule during itsdevelopment. 1441.8.3 Opinio Juris. In addition to analyzing State practice, one must determine whetherthe State practice results from a sense of legal obligation (opinio juris) or merely reflects States’policy or practical interests. Opinio juris cannot simply be inferred from consistent Statepractice, which may exist for reasons other than opinio juris. 145 For example, the fact thatnuclear weapons have not been used to conduct attacks during armed conflict since 1945 doesnot reflect a prohibition in customary international law against their use because such lack of usehas not resulted from opinio juris. 1461.8.3.1 Potential Sources of Opinio Juris. It may be difficult to find evidence ofopinio juris, and care should be exercised in assessing whether a source reflects opinio juris onthe part of a State. For example, treaty provisions do not necessarily reflect opinio juris. 147Similarly, rather than indicating a position expressed out of a sense of a customary legalobligation, a State’s military manual often recites requirements applicable to that State undertreaties to which it is a Party, or provides guidance to its military forces for reasons of nationalpolicy. 148144 Refer to § 1.8.4 (Objection During Development).145 U.S. RESPONSE TO ICRC CIHL STUDY 515 (“Although the same action may serve as evidence both of Statepractice and opinio juris, we do not agree that opinio juris simply can be inferred from practice. Both elementsinstead must be assessed separately in order to determine the presence of a norm of customary international law.”).146 See Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 1996 I.C.J. 226, 253-54 (65-67)(“States which hold the view that the use of nuclear weapons is illegal have endeavoured to demonstrate theexistence of a customary rule prohibiting this use. They refer to a consistent practice of non-utilization of nuclearweapons by States since 1945 and they would see in that practice the expression of an opinio juris on the part ofthose who possess such weapons. Some other States, which assert the legality of the threat and use of nuclearweapons in certain circumstances, invoked the doctrine and practice of deterrence in support of their argument.They recall that they have always, in concert with certain other States, reserved the right to use those weapons in theexercise of the right to self-defence against an armed attack threatening their vital security interests. In their view, ifnuclear weapons have not been used since 1945, it is not on account of an existing or nascent custom but merelybecause circumstances that might justify their use have fortunately not arisen. The Court does not intend topronounce here upon the practice known as the ‘policy of deterrence.’ It notes that it is a fact that a number ofStates adhered to that practice during the greater part of the Cold War and continue to adhere to it. Furthermore, themembers of the international community are profoundly divided on the matter of whether non-recourse to nuclearweapons over the past 50 years constitutes the expression of an opinio juris. Under these circumstances the Courtdoes not consider itself able to find that there is such an opinio juris.”).147 U.S. RESPONSE TO ICRC CIHL STUDY 515 (“One therefore must be cautious in drawing conclusions as to opiniojuris from the practice of States that are parties to conventions, since their actions often are taken pursuant to theirtreaty obligations, particularly inter se, and not in contemplation of independently binding customary internationallaw norms.”).148 U.S. RESPONSE TO ICRC CIHL STUDY 516 (“We are troubled by the Study’s heavy reliance on military manuals.We do not agree that opinio juris has been established when the evidence of a State’s sense of legal obligationconsists predominately of military manuals. Rather than indicating a position expressed out of a sense of acustomary legal obligation, in the sense pertinent to customary international law, a State’s military manual often(properly) will recite requirements applicable to that State under treaties to which it is a party. Reliance on33