Opinion of the
International Court of Justice

On the Legality of Nuclear Weapons

38. The Charter contains several provisions relating to the threat and use of force. In Article 2, paragraph 4, the threat or use of force against the territorial integrity or political independence of another State or in any other manner inconsistent with the purposes of the United Nations is prohibited. That paragraph provides:
"All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the Purposes of the United Nations."

This prohibition of the use of force is to be considered in the light of other relevant provisions of the Charter. In Article 51, the Charter recognizes the inherent right of individual or collective self-defence if an armed attack occurs. A further lawful use of force is envisaged in Article 42, whereby the Security Council may take military enforcement measures in conformity with Chapter VII of the Charter.

39. These provisions do not refer to specific weapons. They apply to any use of force, regardless of the weapons employed. The Charter neither expressly prohibits, nor permits, the use of any specific weapon, including nuclear weapons. A weapon that is already unlawful per se, whether by treaty or custom, does not become lawful by reason of its being used for a legitimate purpose under the Charter.

40. The entitlement to resort to self-defence under Article 51 is subject to certain constraints. Some of these constraints are inherent in the very concept of self defence. Other requirements are specified in Article 51.

41. The submission of the exercise of the right of self-defence to the conditions of necessity and proportionality is a rule of customary international law. As the Court stated in the case concerning Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America) (I.C.J. Reports 1986, p. 94, para. 176): "there is a specific rule whereby self-defence would warrant only measures which are proportional to the armed attack and necessary to respond to it, a rule well established in customary international law". This dual condition applies equally to Article 51 of the Charter, whatever the means of force employed.

42. The proportionality principle may thus not in itself exclude the use of nuclear weapons in self-defence in all circumstances. But at the same time, a use of force that is proportionate under the law of self-defence, must, in order to be lawful, also meet the requirements of the law applicable in armed conflict which comprise in particular the principles and rules of humanitarian law.

43. Certain States have in their written and oral pleadings suggested that in the case of nuclear weapons, the condition of proportionality must be evaluated in the light of still further factors. They contend that the very nature of nuclear weapons, and the high probability of an escalation of nuclear exchanges, mean that there is an extremely strong risk of devastation. The risk factor is said to negate the possibility of the condition of proportionality being complied with. The Court does not find it necessary to embark upon the quantification of such risks; nor does it need to enquire into the question whether tactical nuclear weapons exist which are sufficiently precise to limit those risks: it suffices for the Court to note that the very nature of all nuclear weapons and the profound risks associated therewith are further considerations to be borne in mind by States believing they can exercise a nuclear response in self-defence in accordance with the requirements of proportionality.

44. Beyond the conditions of necessity and proportionality, Article 51 specifically requires that measures taken by States in the exercise of the right of self-defence shall be immediately reported to the Security Council; this article further provides that these measures shall not in any way affect the authority and responsibility of the Security Council under the Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security. These requirements of Article 51 apply whatever the means of force used in self-defence.

45. The Court notes that the Security Council adopted on 11 April 1995, in the context of the extension of the Treaty on the Non-Proliferation of Nuclear Weapons, resolution 984 (1995) by the terms of which, on the one hand, it
"[t]akes note with appreciation of the statements made by each of the nuclear-weapon States (S/1995/261, S/1995/262, S/1995/263, S/1995/264, S/1995/265), in which they give security assurances against the use of nuclear weapons to non-nuclear-weapon States that are Parties to the Treaty on the Non-Proliferation of Nuclear Weapons," and, on the other hand, it "[w]elcomes the intention expressed by certain States that they will provide or support immediate assistance, in accordance with the Charter, to any non-nuclear-weapon State Party to the Treaty on the Non-Proliferation of Nuclear Weapons that is a victim of an act of, or an object of a threat of, aggression in which nuclear weapons are used".

46. Certain States asserted that the use of nuclear weapons in the conduct of reprisals would be lawful. The Court does not have to examine, in this context, the question of armed reprisals in time of peace, which are considered to be unlawful. Nor does it have to pronounce on the question of belligerent reprisals save to observe that in any case any right of recourse to such reprisals would, like self-defence, be governed inter alia by the principle of proportionality.

47. In order to lessen or eliminate the risk of unlawful attack, States sometimes signal that they possess certain weapons to use in self-defence against any State violating their territorial integrity or political independence. Whether a signalled intention to use force if certain events occur is or is not a "threat" within Article 2, paragraph 4, of the Charter depends upon various factors. If the envisaged use of force is itself unlawful, the stated readiness to use it would be a threat prohibited under Article 2, paragraph 4. Thus it would be illegal for a State to threaten force to secure territory from another State, or to cause it to follow or not follow certain political or economic paths. The notions of "threat" and "use" of force under Article 2, paragraph 4, of the Charter stand together in the sense that if the use of force itself in a given case is illegal for whatever reason the threat to use such force will likewise be illegal. In short, if it is to be lawful, the declared readiness of a State to use force must be a use of force that is in conformity with the Charter. For the rest, no State whether or not it defended the policy of deterrence suggested to the Court that it would be lawful to threaten to use force if the use of force contemplated would be illegal.

48. Some States put forward the argument that possession of nuclear weapons is itself an unlawful threat to use force. Possession of nuclear weapons may indeed justify an inference of preparedness to use them. In order to be effective, the policy of deterrence, by which those States possessing or under the umbrella of nuclear weapons seek to discourage military aggression by demonstrating that it will serve no purpose, necessitates that the intention to use nuclear weapons be credible. Whether this is a "threat" contrary to Article 2, paragraph 4, depends upon whether the particular use of force envisaged would be directed against the territorial integrity or political independence of a State, or against the Purposes of the United Nations or whether, in the event that it were intended as a means of defence, it would necessarily violate the principles of necessity and proportionality. In any of these circumstances the use of force, and the threat to use it, would be unlawful under the law of the Charter.

49. Moreover, the Security Council may take enforcement measures under Chapter VII of the Charter. From the statements presented to it the Court does not consider it necessary to address questions which might, in a given case, arise from the application of Chapter VII.

50. The terms of the question put to the Court by the General Assembly in resolution 49/75K could in principle also cover a threat or use of nuclear weapons by a State within its own boundaries. However, this particular aspect has not been dealt with by any of the States which addressed the Court orally or in writing in these proceedings. The Court finds that it is not called upon to deal with an internal use of nuclear weapons.

51. Having dealt with the Charter provisions relating to the threat or use of force, the Court will now turn to the law applicable in situations of armed conflict. It will first address the question whether there are specific rules in international law regulating the legality or illegality of recourse to nuclear weapons per se; it will then examine the question put to it in the light of the law applicable in armed conflict proper, i.e. the principles and rules of humanitarian law applicable in armed conflict, and the law of neutrality.

52. The Court notes by way of introduction that international customary and treaty law does not contain any specific prescription authorizing the threat or use of nuclear weapons or any other weapon in general or in certain circumstances, in particular those of the exercise of legitimate self-defence. Nor, however, is there any principle or rule of international law which would make the legality of the threat or use of nuclear weapons or of any other weapons dependent on a specific authorization. State practice shows that the illegality of the use of certain weapons as such does not result from an absence of authorization but, on the contrary, is formulated in terms of prohibition.

53. The Court must therefore now examine whether there is any prohibition of recourse to nuclear weapons as such; it will first ascertain whether there is a conventional prescription to this effect.

54. In this regard, the argument has been advanced that nuclear weapons should be treated in the same way as poisoned weapons. In that case, they would be prohibited under:
(a) the Second Hague Declaration of 29 July 1899, which prohibits "the use of projectiles the object of which is the diffusion of asphyxiating or deleterious gases";

(b) Article 23 (a) of the Regulations respecting the laws and customs of war on land annexed to the Hague Convention IV of 18 October 1907, whereby "it is especially forbidden: ...to employ poison or poisoned weapons"; and

(c) the Geneva Protocol of 17 June 1925 which prohibits "the use in war of asphyxiating, poisonous or other gases, and of all analogous liquids, materials or devices".

55. The Court will observe that the Regulations annexed to the Hague Convention IV do not define what is to be understood by "poison or poisoned weapons" and that different interpretations exist on the issue. Nor does the 1925 Protocol specify the meaning to be given to the term "analogous materials or devices". The terms have been understood, in the practice of States, in their ordinary sense as covering weapons whose prime, or even exclusive, effect is to poison or asphyxiate. This practice is clear, and the parties to those instruments have not treated them as referring to nuclear weapons.

56. In view of this, it does not seem to the Court that the use of nuclear weapons can be regarded as specifically prohibited on the basis of the above-mentioned provisions of the Second Hague Declaration of 1899, the Regulations annexed to the Hague Convention IV of 1907 or the 1925 Protocol (see paragraph 54 above).

57. The pattern until now has been for weapons of mass destruction to be declared illegal by specific instruments. The most recent such instruments are the Convention of 10 April 1972 on the Prohibition of the Development, Production and Stockpiling of Bacteriological (Biological) and Toxin Weapons and on their destruction which prohibits the possession of bacteriological and toxic weapons and reinforces the prohibition of their use and the Convention of 13 January 1993 on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on Their Destruction which prohibits all use of chemical weapons and requires the destruction of existing stocks. Each of these instruments has been negotiated and adopted in its own context and for its own reasons. The Court does not find any specific prohibition of recourse to nuclear weapons in treaties expressly prohibiting the use of certain weapons of mass destruction.

58. In the last two decades, a great many negotiations have been conducted regarding nuclear weapons; they have not resulted in a treaty of general prohibition of the same kind as for bacteriological and chemical weapons. However, a number of specific treaties have been concluded in order to limit:
(a) the acquisition, manufacture and possession of nuclear weapons (Peace Treaties of 10 February 1947; State Treaty for the Re-establishment of an Independent and Democratic Austria of 15 May 1955; Treaty of Tlatelolco of 14 February 1967 for the Prohibition of Nuclear Weapons in Latin America, and its Additional Protocols; Treaty of 1 July 1968 on the Non-Proliferation of Nuclear Weapons; Treaty of Rarotonga of 6 August 1985 on the Nuclear- Weapon-Free Zone of the South Pacific, and its Protocols; Treaty of 12 September 1990 on the Final Settlement with respect to Germany);

(b) the deployment of nuclear weapons (Antarctic Treaty of 1 December 1959; Treaty of 27 January 1967 on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies; Treaty of Tlatelolco of 14 February 1967 for the Prohibition of Nuclear Weapons in Latin America, and its Additional Protocols; Treaty of 11 February 1971 on the Prohibition of the Emplacement of Nuclear Weapons and Other Weapons of Mass Destruction on the Sea-Bed and the Ocean Floor and in the Subsoil Thereof; Treaty of Rarotonga of 6 August 1985 on the Nuclear-Weapon-Free Zone of the South Pacific, and its Protocols); and

(c) the testing of nuclear weapons (Antarctic Treaty of 1 December 1959; Treaty of 5 August 1963 Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and under Water; Treaty of 27 January 1967 on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies; Treaty of Tlatelolco of 14 February 1967 for the Prohibition of Nuclear Weapons in Latin America, and its Additional Protocols; Treaty of Rarotonga of 6 August 1985 on the Nuclear- Weapon-Free Zone of the South Pacific, and its Protocols).

59. Recourse to nuclear weapons is directly addressed by two of these Conventions and also in connection with the indefinite extension of the Treaty on the Non-Proliferation of Nuclear Weapons of 1968:
(a) the Treaty of Tlatelolco of 14 February 1967 for the Prohibition of Nuclear Weapons in Latin America prohibits, in Article 1, the use of nuclear weapons by the Contracting Parties. It further includes an Additional Protocol II open to nuclear-weapon States outside the region, Article 3 of which provides:
"The Governments represented by the undersigned Plenipotentiaries also undertake not to use or threaten to use Nuclear weapons against the Contracting Parties of the Treaty for the Prohibition of Nuclear Weapons in Latin America."
The Protocol was signed and ratified by the five nuclear-weapon States. Its ratification was accompanied by a variety of declarations. The United Kingdom Government, for example, stated that "in the event of any act of aggression by a Contracting Party to the Treaty in which that Party was supported by a nuclear-weapon State", the United Kingdom Government would "be free to reconsider the extent to which they could be regarded as committed by the provisions of Additional Protocol II". The United States made a similar statement. The French Government, for its part, stated that it "interprets the undertaking made in article 3 of the Protocol as being without prejudice to the full exercise of the right of self-defence confirmed by Article 51 of the Charter". China reaffirmed its commitment not to be the first to make use of nuclear weapons. The Soviet Union reserved "the right to review" the obligations imposed upon it by Additional Protocol II, particularly in the event of an attack by a State party either "in support of a nuclear-weapon State or jointly with that State". None of these statements drew comment or objection from the parties to the Treaty of Tlatelolco.

(b) the Treaty of Rarotonga of 6 August 1985 establishes a South Pacific Nuclear Free Zone in which the Parties undertake not to manufacture, acquire or possess any nuclear explosive device (Art. 3). Unlike the Treaty of Tlatelolco, the Treaty of Rarotonga does not expressly prohibit the use of such weapons. But such a prohibition is for the States parties the necessary consequence of the prohibitions stipulated by the Treaty. The Treaty has a number of protocols. Protocol 2, open to the five nuclear-weapon States, specifies in its Article 1 that:
"Each Party undertakes not to use or threaten to use any nuclear explosive device against:
(a) Parties to the Treaty; or

(b) any territory within the South Pacific Nuclear Free Zone for which a State that has become a Party to Protocol 1 is internationally responsible."
China and Russia are parties to that Protocol. In signing it, China and the Soviet Union each made a declaration by which they reserved the" right to reconsider" their obligations under the said Protocol; the Soviet Union also referred to certain circumstances in which it would consider itself released from those obligations. France, the United Kingdom and the United States, for their part, signed Protocol 2 on 25 March 1996, but have not yet ratified it. On that occasion, France declared, on the one hand, that no provision in that Protocol "shall impair the full exercise of the inherent right of self-defence provided for in Article 51 of the ... Charter" and, on the other hand, that "the commitment set out in Article 1 of [that] Protocol amounts to the negative security assurances given by France to non-nuclear-weapon States which are parties to the Treaty on . . . Non-Proliferation", and that "these assurances shall not apply to States which are not parties" to that Treaty. For its part, the United Kingdom made a declaration setting out the precise circumstances in which it "will not be bound by [its] undertaking under Article 1" of the Protocol.

(c) as to the Treaty on the Non-Proliferation of Nuclear Weapons, at the time of its signing in 1968 the United States, the United Kingdom and the USSR gave various security assurances to the non-nuclear-weapon States that were parties to the Treaty. In resolution 255 (1968) the Security Council took note with satisfaction of the intention expressed by those three States to
"provide or support immediate assistance, in accordance with the Charter, to any non-nuclear-weapon State Party to the Treaty on the Non-Proliferation . . . that is a victim of an act of, or an object of a threat of, aggression in which nuclear weapons are used".
On the occasion of the extension of the Treaty in 1995, the five nuclear-weapon States gave their non-nuclear-weapon partners, by means of separate unilateral statements on 5 and 6 April 1995, positive and negative security assurances against the use of such weapons. All the five nuclear- weapon States first undertook not to use nuclear weapons against non-nuclear-weapon States that were parties to the Treaty on the Non-Proliferation of Nuclear Weapons. However, these States, apart from China, made an exception in the case of an invasion or any other attack against them, their territories, armed forces or allies, or on a State towards which they had a security commitment, carried out or sustained by a non-nuclear-weapon State party to the Non-Proliferation Treaty in association or alliance with a nuclear-weapon State. Each of the nuclear-weapon States further undertook, as a permanent Member of the Security Council, in the event of an attack with the use of nuclear weapons, or threat of such attack, against a non-nuclear-weapon State, to refer the matter to the Security Council without delay and to act within it in order that it might take immediate measures with a view to supplying, pursuant to the Charter, the necessary assistance to the victim State (the commitments assumed comprising minor variations in wording). The Security Council, in unanimously adopting resolution 984 (1995) of 11 April 1995, cited above, took note of those statements with appreciation. It also recognized
"that the nuclear-weapon State permanent members of the Security Council will bring the matter immediately to the attention of the Council and seek Council action to provide, in accordance with the Charter, the necessary assistance to the State victim";
and welcomed the fact that
"the intention expressed by certain States that they will provide or support immediate assistance, in accordance with the Charter, to any non-nuclear- weapon State Party to the Treaty on the Non-Proliferation of Nuclear Weapons that is a victim of an act of, or an object of a threat of, aggression in which nuclear weapons are used."

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