The Inadmissibility of Nuclear Threats

April 2024 
By John Burroughs

Contradicting the widespread and complacent post-Cold War belief that the risks of the nuclear age are declining, threats of the possible use of nuclear weapons are on the rise.

Homes and trees destroyed by Russian shelling in the Zaporizhzhia region of Ukraine create a haunting landscape in March as Moscow’s full-scale invasion of the country grinds into its third year. On numerous occasions, Russian President Vladimir Putin has raised the possibility of using nuclear weapons if the United States and NATO intervene to defend Ukraine.  (Photo by Andriy Andriyenko/SOPA Images/LightRocket via Getty Images)In the summer and autumn of 2017, the United States and North Korea exchanged incendiary warnings of nuclear destruction.1 In September 2019, Pakistan referred to possible nuclear war in connection to its dispute with India over Kashmir.2 In recent months, North Korean leader Kim Jong Un on several occasions reiterated his country’s readiness to resort to nuclear arms to defend its fundamental interests.3 Most alarmingly, the Russian government on numerous occasions, beginning with President Vladimir Putin’s speech on February 24, 2022,4 and up to the present, has raised the possibility of resorting to nuclear weapons should the United States and its NATO allies intervene to defend Ukraine against the full-scale Russian invasion.

Such threats are utterly unacceptable, above all because they greatly increase the risks of a humanitarian and environmental catastrophe resulting from use of nuclear weapons. The position adopted by the Group of 20 (G-20) states, an intergovernmental forum that includes the world’s major powers, in a declaration in Bali in November 2022 is striking in this regard. The declaration states in part, “It is essential to uphold international law and the multilateral system that safeguards peace and stability. This includes defending all the Purposes and Principles enshrined in the Charter of the United Nations and adhering to international humanitarian law, including the protection of civilians and infrastructure in armed conflicts. The use or threat of use of nuclear weapons is inadmissible.”5

Although that position clearly was occasioned by Russian nuclear threats, by its terms, it is not limited to that circumstance.6 It was repeated in another G-20 declaration in New Delhi in September 2023.

The Legal Dimension

Is a declaration of the “inadmissibility” of the threat and the use of nuclear arms an articulation of a political and moral norm, or does it also have a legal dimension? After all, the reference to inadmissibility does have a legal flavor; in a trial, evidence is found to be admissible or inadmissible. Further, there is a strong case to be made that threats of nuclear weapons use are not only unacceptable and illegitimate but contrary to international law. That is true under law governing when the resort to force is lawful (jus ad bellum) and under law governing the conduct of conflict (jus in bello), the law of armed conflict or international humanitarian law.

Article 2(4) of the UN Charter provides that “[a]ll 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.” If a use of force would violate Article 2(4), a threat to engage in such force violates that article. As the International Court of Justice (ICJ) stated broadly in its 1996 nuclear weapons advisory opinion, “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.”7

It follows that a threat to use nuclear weapons as part of an aggressive attack is illegal. That certainly applies to the nuclear threats made in support of Russia’s invasion of Ukraine.

What is the law, though, when a state threatens to use nuclear arms not as part of an aggressive attack as in the Russian case? As the ICJ explained, the use or threat of force in self-defense pursuant to Article 51 of the Charter must be necessary and proportional.8 A defensive threat to use nuclear weapons that does not meet those criteria would be illegal under jus ad bellum. In this context,9 proportionality requires that the defensive use of force have a reasonable relationship to the aggressive act responded to and a reasonable relationship to the lawful goals of the defensive use of force, for example, expelling troops from the attacked state’s territory. In many or all circumstances, a defensive first use of nuclear weapons would not be necessary or proportionate as a matter of jus ad bellum. Notably, the ICJ observed that the risk of escalation must be taken into account in assessing proportionality.10 Recent North Korean nuclear threats fail to meet the requirement of proportionality.

The court also found that any threat to use nuclear weapons, whether aggressive or defensive, must be of a use that would comply with international humanitarian law or jus in bello. In general, the ICJ said that “[i]f an envisaged use of weapons would not meet the requirements of humanitarian law, a threat to engage in such use would also be contrary to that law.”11 Under that principle, if a use of nuclear arms is illegal, the threat of their use is illegal.

Putting aside marginal cases, nuclear use in typical scenarios, even if defensive, would be illegal under international humanitarian law.12 The ICJ went a long way toward accepting this conclusion, finding that “the threat or use of nuclear weapons would generally be contrary to the rules of international law applicable in armed conflict, and in particular the principles and rules of humanitarian law.”13 The court did not reach a conclusion, one way or the other, however, regarding an extreme circumstance of self-defense in which the very survival of a state is at stake.14 Nearly three decades after the court issued its opinion, the global community should move beyond the court’s uncertainty in that circumstance.

The ICJ Analysis of Threat

The ICJ analysis of UN Charter requirements barring aggressive or disproportionate threats of force is unexceptionable; it flows from the Charter and the well-established rule that a defensive use of force must be necessary and proportional. In contrast, the basis for the court’s finding that a threat to use weapons in violation of international humanitarian law is illegal is not clear from the advisory opinion, nor is such a principle readily ascertainable in treaty law or ICJ case law.

Yet, the proposition that threats of illegal force are themselves illegal is rooted in the most important modern treaty, the UN Charter. Although the Charter does not address directly the issue of whether the illegality of threats of illegal force extends to violations of international humanitarian law, it does imply that the legality of threat of force and the legality of use of force should be analyzed together. The court’s reference under Article 2(4) to the illegality of threats of uses of force when the latter are illegal “for whatever reason” is consistent with that implication.15 Moreover, the Article 2(4) prohibition of the threat of force inconsistent with the purposes of the UN provides some support for an analysis going beyond the prohibitions of aggressive or disproportionate threats. Purposes set out in Article 1 include the maintenance of peace and security and cooperation in solving problems of a humanitarian character and in promoting respect for human rights.

Beyond those Charter considerations, it appears that the court enunciated the principle that a threat to use weapons in violation of international humanitarian law is illegal on its own authority. The ICJ is the highest court in the world on general questions of international law, and its judges include eminent international lawyers. It is not unusual for the highest court in a judicial system to develop the law or, put another way, to make visible already existing principles.

Sometimes, there is a distinction made in international law between conduct preparatory to a wrongful act and the wrongful act itself, which is illegal or criminal. This tendency is visible in the crime of aggression included in the Rome Statute of the International Criminal Court, which provides that an individual may be convicted of “planning” or “prepar[ing]” for aggression, but only if a state “act of aggression” is actually committed. If this approach were taken in the realm of state responsibility, a threat would be illegal only if contrary to a treaty, as it is in the case of threats of aggression under the Charter.16

A threat to use weapons in violation of international humanitarian law, certainly a specific and credible threat, is different from preparatory conduct such as acquiring military capabilities enabling an aggressive attack. A thought exercise regarding biological and chemical weapons illustrates the soundness of the court’s finding. The use of biological weapons and the use of chemical weapons would be violations of international humanitarian law prohibitions of attacks with indiscriminate and uncontrollable effects.17 Further, a nearly universal convention, the Chemical Weapons Convention, prohibits the possession and use of chemical arms; another one, the Biological Weapons Convention, prohibits possession of biological arms, reinforcing an existing ban on their use. Should a specific and credible threat to use such arms be considered lawful? Going beyond the sphere of weapons, under the Genocide Convention, incitement, conspiracy, and attempt to commit genocide are prohibited but not threats. Yet, it seems highly doubtful that a specific and credible threat to commit genocide should be considered lawful.

There are partial international prohibitions of threat under international humanitarian law.18 A key instrument, Additional Protocol I to the 1949 Geneva Conventions, includes a provision prohibiting “acts or threats of violence the primary purpose of which is to spread terror among the civilian population.”19 Another provision prohibits threatening that there shall be no survivors.20

Such prohibitions can be viewed in two ways. One is that their partial character demonstrates the lack of a comprehensive prohibition of threats to use weapons in violation of international humanitarian law. The second is that they are expressions of an underlying principle, namely, that it is prohibited to threaten to carry out a prohibited act.21

The Evolution of International Law

The latter view is more consonant with the evolution of international law, recently illustrated by the 2017 negotiation of the Treaty on the Prohibition of Nuclear Weapons (TPNW) and the UN Human Rights Committee’s 2018 General Comment on the right to life. In that comment, the committee found that “[t]he threat or use of weapons of mass destruction, in particular nuclear weapons…is incompatible with respect for the right to life and may amount to a crime under international law.”22

The TPNW is the latest manifestation of the view of a majority of the world’s states that the threat or use of nuclear arms is illegal. Previously, such views were expressed in arguments to the ICJ and, regarding use, in numerous UN General Assembly resolutions going back to 1961.23 Moreover, beginning with the 1967 Treaty of Tlatelolco, non-nuclear-weapon states have negotiated regional nuclear-weapon-free-zone treaties with protocols that, when ratified, obligate the five nuclear-weapon states acknowledged by the nuclear Nonproliferation Treaty not to use or threaten to use nuclear arms against members of the regional zones.24

Despite nuclear saber-rattling in recent years by Russia, North Korea, and others, the entry-into-force of the Treaty on the Prohibition of Nuclear Weapons (TPNW) shows that a majority of the world’s states view the threat or use of nuclear weapons as illegal. As TPNW states-parties held their second meeting in November in New York, Juan Ramon de la Fuente of Mexico (C, R), the meeting president, spoke with delegates. (Photo by ICAN/Darren Ornitz)In June 2022, five months prior to the adoption of the G-20 declaration in Bali, the first meeting of TPNW states-parties adopted the Vienna Declaration. In it, states-parties “stress that any use or threat of use of nuclear weapons is a violation of international law”; and they “condemn unequivocally any and all nuclear threats, whether they be explicit or implicit and irrespective of the circumstances.” The second meeting of states-parties, in New York City in late 2023, reiterated those points. The TPNW itself obligates states-parties “never under any circumstances” to “use or threaten to use nuclear weapons.” TPNW states-parties do not include any nuclear-armed states.

These developments further support the soundness of the court’s findings: threats to use weapons that violate international humanitarian law and therefore, at least as a general matter, threats to use nuclear weapons are illegal. It bears repeating as well that threats to use disproportionate force in self-defense are illegal, and the risk of escalation, obviously an acute concern when it comes to nuclear arms, must be taken into account in assessing proportionality.

Without attempting a definition of “threat” as a matter of international law, it can safely be stated that a specific and credible governmental statement making demands qualifies. Take a concrete context where the stakes are high, such as an armed conflict involving a nuclear-armed state; and the message is, if you do not refrain from doing X or if you do Y, we will resort to nuclear arms. That undoubtedly is a legally cognizable threat. It certainly describes Putin’s threat at the outset of the invasion of Ukraine, in which he expressed a readiness to resort to nuclear force should NATO states “interfere” in Russian military operations in Ukraine.

What about standing policies declaring a state’s readiness to resort to nuclear weapons when subjected to a nuclear attack and more generally when vital interests are stake? One could argue that specific threats are not involved in those policies and therefore they are not illegal. It is true that references to vital interests and similar formulations are vague. Yet, doctrinal signals that a nuclear attack may or will be met with a responsive or preemptive nuclear attack are focused and credible, even if not issued in an actual circumstance of potential use.

More broadly, if specific threats are illegal, that points toward the illegality and certainly the illegitimacy of the machinery and doctrines of nuclear deterrence. In its advisory opinion, the ICJ stated that it “does not intend to pronounce here upon the practice known as the ‘policy of deterrence.’”25 Yet, as the United States observed in its oral argument to the court, “it is impossible to separate the policy of deterrence from the legality of the use of the means of deterrence.”26

It is highly disturbing that nuclear threats are on the rise, in the Russian-Ukrainian conflict and elsewhere. Even so, there are positive trends, although their importance should not be overstated. It is encouraging that there has been a counterassertion of a norm against nuclear threats in G-20 summits, as well as in the TPNW meetings. It is also encouraging that the United States and other NATO states for the most part have refrained from any public nuclear threats in response to those made by Russia.

The norm against threatening the use of nuclear weapons has a firm legal foundation. It is imperative that governments and civil society strive to entrench it even more deeply in international and national understanding and practice.



1. John Burroughs and Andrew Lichterman, “Trump’s Threat of Total Destruction Is Unlawful and Extremely Dangerous,” InterPress Service, September 25, 2017.

2. “Pakistan’s Khan Warns of All-Out Conflict Amid Rising Tensions Over Kashmir; Demands India Lift ‘Inhuman’ Curfew,” UN News, September 27, 2019, Earlier in the year, Indian Prime Minister Narendra Modi referred to India’s nuclear weapons capability in relation to tensions with Pakistan. Pallak Nandi and Vimal Bhatia, “Our nukes not for Diwali: PM Narendra Modi on Pakistan's N-threat,” The Times of India, April 22, 2019,

3. Josh Smith, “Explainer: How Could North Korea Use Its Nuclear Weapons?” Reuters, December 20, 2023.

4. From the transcript provided by the Kremlin: “I would now like to say something very important for those who may be tempted to interfere in these developments from the outside. No matter who tries to stand in our way or all the more so create threats for our country and our people, they must know that Russia will respond immediately, and the consequences will be such as you have never seen in your entire history,”

5.  “G20 Bali Leaders’ Declaration,” November 15-16, 2022, para. 4, http://www.g20.utoronto 

6. A February 24, 2024, declaration by the Group of Seven states (Canada, France, Germany, Italy, Japan, the United Kingdom, and the United States) is limited in its scope, stating, “Threats by Russia of nuclear weapon use, let alone any use of nuclear weapons by Russia, in the context of its war of aggression against Ukraine are inadmissible.” The White House, “G7 Leaders’ Statement,” February 24, 2024, para. 2,

7.  Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 1996 I.C.J. 226, para. 47 (July 8),

8. Ibid., para. 41.

9. The jus ad bellum requirement of proportionality differs from the international humanitarian law principle of proportionality, which requires that the collateral damage and injury resulting from an attack not be excessive in relation to the anticipated military advantage.

10. Legality of the Threat or Use of Nuclear Weapons, 1996 I.C.J. at para. 43.

11. Ibid., para. 78.

12. See “End the War, Stop the War Crimes,” Lawyers Committee on Nuclear Policy, April 21, 2022, pp. 5-6,; Charles J. Moxley Jr., John Burroughs, and Jonathan Granoff, “Nuclear Weapons and Compliance With International Humanitarian Law and the Nuclear Non-Proliferation Treaty,” Fordham International Law Journal, Vol. 34, No. 4 
(2011): 595.

13. Legality of the Threat or Use of Nuclear Weapons, 1996 I.C.J. at para. 105(2)E.

14. Ibid.

15. Ibid., para. 47.

16. The International Law Commission has taken this position, stating, “Some rules specifically prohibit threats of conduct, incitement or attempt, in which case the threat, incitement or attempt is itself a wrongful act.” Yearbook of the International Law Commission, 2001, Vol. 2, Part 2, A/CN.4/SER.A/2001/Add.1 (Part 2), 2007, p. 61.

17. In addition to general rules of international humanitarian law, an amendment to the Rome Statute, so far ratified by only a small number of states, specifically makes use of biological weapons a war crime. Article 8(2)(b)(xxvii). Use of chemical weapons arguably is specifically criminal under a provision of the Rome Statute setting forth the Geneva Gas Protocol prohibition of use of “asphyxiating, poisonous or other gases, and all analogous liquids, materials or devices.” Article 8(2)(b)(xviii).

18. Principles embedded in national legal systems are also relevant to determination of general principles of international law. U.S. jurisdictions, for example, prohibit threat or menacing in certain contexts. Thus a Colorado statute provides that “[a] person commits the crime of menacing if, by any threat or physical action, he or she knowingly places or attempts to place another person in fear of imminent serious bodily injury.” Colo. Rev. Stat. § 18-3-206. Under federal law, transmitting “in interstate or foreign commerce any communication containing any threat to kidnap any person or any threat to injure the person of another” is a crime. 18 U.S.C. § 875(c).

19. Protocol Additional to the Geneva Conventions of 12 August 1949, and Relating to the Protection of Victims of International Armed Conflicts (Protocol I), June 8, 1977, 1125 U.N.T.S. 3, art. 51(2). The U.S. 2022 Nuclear Posture Review states that “longstanding U.S. policy is to not purposely threaten civilian populations and objects.” U.S. Department of Defense, “2022 Nuclear Posture Review,” October 27, 2022, p. 8, Taken on its face and separated from the realities of targeting military objectives, that statement is consistent with the prohibition.

20. Protocol Additional to the Geneva Conventions of 12 August 1949, art. 40.

21. Jean-Marie Henckaerts and Louise Doswald-Beck, Customary International Humanitarian Law, Vol. I (Cambridge: Cambridge University Press, 2005), p. 162.

22. UN Human Rights Committee, “General Comment No. 36 (2018) on Article 6 of the International Covenant on Civil and Political Rights, on the Right to Life,” CCPR/C/GC/36, October 30, 2018, para. 66.

23. See UN General Assembly, “Declaration on the Prohibition of the Use of Nuclear and Thermo-Nuclear Weapons,” Res. 1653 (XVI), November 24, 1961.

24. Nuclear-weapon states’ ratifications of the protocols are far from complete and in some cases are heavily qualified. Also, within and outside the nuclear Nonproliferation Treaty, nuclear-armed states have provided negative security assurances, qualified in most cases, that they will not use and in some cases not threaten to use nuclear arms against non-nuclear-weapon states. The International Court of Justice unsurprisingly found that nuclear-weapon-free-zone treaties and their protocols and negative security assurances do not establish a comprehensive prohibition of the threat or use of nuclear weapons. Legality of the Threat or Use of Nuclear Weapons, 1996 I.C.J. at para. 63. See generally Anna Hood and Monique Cormier, “Nuclear Threats Under International Law Part 1: The Legal Framework,” NAPSNet Special Reports, March 1, 2024.

25. Legality of the Threat or Use of Nuclear Weapons, 1996 I.C.J. at para. 67.

26. Verbatim record, CR 95/34, November 15, 1995, p. 63,


John Burroughs is senior analyst for Lawyers Committee on Nuclear Policy and author of The Legality of Threat or Use of Nuclear Weapons: A Guide to the Historic Opinion of the International Court of Justice. This article draws on the author’s remarks at the conference “Nuclear Weapons and International Law: The Renewed Imperative in Light of the Russian Invasion of Ukraine” on November 8, 2023.