On October 24, 2020, following Honduras’ ratification, the UN announced the Treaty on the Prohibition of Nuclear Weapons (TPNW) would enter into force on January 22, 2021. Accompanying this announcement, the UN Secretary General’s spokesman asserted the soon-to-be legally operative TPNW “is the culmination of a worldwide movement to draw attention to the catastrophic humanitarian consequences of any use of nuclear weapons. It represents a meaningful commitment towards the total elimination of nuclear weapons, which remains the highest disarmament priority of the United Nations.” The Executive Director of the International Campaign to Abolish Nuclear Weapons (ICAN) celebrated the treaty’s milestone moment by claiming, “This is a new chapter for nuclear disarmament. Decades of activism have achieved what many said was impossible: nuclear weapons are banned.”
Asserting in its preamble that any use of nuclear weapons would already be “contrary to the rules of international law applicable in armed conflict,” the TPNW will be the first legally binding treaty that completely prohibits their use. Of course, ratifying states will be bound by that ban. However, for states not party to the TPNW including the United States and all other nuclear-armed states, this moment begs the question of whether the treaty is, as suggested in the four corners of the instrument itself, a reflection of an underlying customary international law norm that proscribes the use of nuclear weapons. Setting aside the moral aspects surrounding the use of nuclear weapons, in our view, it is not.
While the fiftieth state’s ratification of the TPNW is certainly a momentous occasion for those who support the admirable goal of eradicating nuclear weapons, the moment is, unfortunately, more symbolic than it is indicative of a customary proscription on their use.
What is the TPNW?
The TPNW was adopted during a UN General Assembly conference on July 7, 2017. Among the treaty’s provisions are prohibitions on developing, testing, producing, manufacturing, otherwise acquiring, possessing, stockpiling, transferring, and receiving nuclear weapons. It further bars states from assisting, encouraging or inducing anyone to engage in any activity prohibited by the treaty, and seeking or receiving any assistance, in any way, from anyone to engage in activity prohibited under the treaty. The treaty also prohibits states parties from allowing another state to station, install, or deploy nuclear weapons in its territory. Most notably, the treaty completely bans using or threatening to use nuclear weapons. Simply put, the treaty seeks the total elimination of nuclear weapons to ensure they are never used again.
Currently, eighty-four states are signatories, with fifty states having ratified the TPNW at the time of this writing. Significantly, no state with nuclear capability is a signatory to the treaty, nor has any NATO member, or any other state that benefits from a nuclear umbrella agreement, signed it. In fact, the Netherlands was the only NATO member that participated in the treaty negotiations leading to its adoption. The United States has urged other states to withdraw their support for the TPNW.
Treaties and Customary International Law
Article 38 of the Statute of the International Court of Justice, a provision with which all states agree, characterizes “a general practice accepted as law” as a primary source of law, together with treaties and general principles of law. These binding norms are known as “customary international law,” which emerge through “extensive and virtually uniform” state practice engaged in, or refrained from, out of a sense of legal obligation (opinio juris). While treaties are a distinct primary source of law, in certain circumstances, treaty provisions may be reflective of existing customary international law, “or indeed [of] developing [new customary norms].” The question is whether a proscription on the use of nuclear weapons is one such rule.
The terms of treaties predating the TPNW demonstrate the lack of a customary prohibition on the use of nuclear weapons at the time they were adoped. Consider the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), which entered into force in March 1970. Today, there are over 190 states party to the agreement. Under the NPT, non-nuclear states agreed not to receive, transfer, manufacture, or otherwise acquire nuclear weapons. Nuclear states agreed not to transfer nuclear weapons to, or otherwise assist, or encourage non-nuclear weapon states in acquiring nuclear weapons. Five nuclear weapon states are party to the NPT—China, France, Russia, the United Kingdom, and the United States. They are also the five permanent members of the UN Security Council. Other states parties do not have nuclear weapon capabilities and are prohibited from gaining those capabilities.
The NPT also led the nuclear powers to agree to a no-use commitment that obligates them to refrain from using nuclear weapons against non-nuclear states parties and to assist states that are attacked by nuclear weapons. With the exception of China, they made an exception to the no-use commitment “in the case of an invasion or any other attack against them” carried out by a non-nuclear state that is supported by a nuclear power. In May 1995, the NPT was extended indefinitely, with China, France, Russia, the United Kingdom and the United States all reconfirming their prior no-use commitments.
The text of the NPT and the resulting no-use commitments by the five nuclear powers evince that at least as late as 1995 there was no customary norm prohibiting the use of nuclear weapons. The very existence of the treaty recognizing retention of the weapons by some states is inconsistent with a customary rule proscribing use. And while Article VI contemplates the pursuit of negotiations toward the potential complete disarmament in the future, it does not require it. Obviously, a treaty cannot affirmatively allow for possession of an “illegal to use” weapon. Moreover, the exceptions to the negotiated no-use commitments illustrate that states parties did not view the use of nuclear weapons as an unlawful act.
The practice of the few remaning states that are not party to the NPT also contradict the existence of a customary rule proscribing nuclear weapons. Since the treaty entered into force, at least two states that are not party to the treaty (India and Pakistan) obtained nuclear weapon capabilities. Israel is also reported to have them, and North Korea, which withdrew from the NPT in 2003, has since openly tested its nuclear weapon capabilities (While North Korea’s actions have been widely condemned by the international community, the condemnation is more indicative of the fact North Korea was not acting in good faith when it withdrew from the treaty rather than the emergence of a customary norm).
In addition to the NPT, there are many other multilateral treaties regulating the use of nuclear weapons. For instance, there are numerous regional nuclear-weapon-free zone treaties. These treaties prohibit the use of nuclear weapons within certain regions, such as Antartica, Africa, South & Central America, the South Pacific, outer space, and the ocean floor. The language of these treaties arguably suggests that states parties recognize there is no blanket customary norm prohibiting the use of nuclear weapons and therefore they have to carve out specific regions where use is prohibited by treaty. As the International Court of Justice (ICJ) concluded in its Legality of the Threat or Use of Nuclear Weapons advisory opinion, “these treaties could  be seen as foreshadowing a future general prohibition of the use of [nuclear weapons], but they do not constitute such a prohibition by themselves.”
State Practice and Contrary Opinio Juris
Turning to state practice—both physical and verbal acts—it is clear that a comprehensive prohibition on using nuclear weapons has not emerged as customary international law. Particularly relevant in this regard is the practice of states “whose interests [are] specially affected”—those with nuclear capabilities and, thus, most likely to have those weapons used against them. And “evidence of a belief that [the relevant] practice is rendered obligatory by the existence of a rule of law requiring it” (opinio juris) is needed in addition to state practice before crystallization into a binding customary rule of international law occurs.
The United States and other nuclear weapon states have all adopted a “policy of deterrence,” with some having pledged to a no-first-use policy. Nevertheless, they have all displayed a credible willingness to use the weapons should deterrence fail. This strategic posture, grounded in political and practical considerations, appears to haved worked, for it has been over seventy years since any state employed a nuclear weapon. Yet, that non-use does not qualify as state practice contributing to the emergence of a prohibition because it was accompanied by the continued possession of the weapons and statements of willingness to employ them.
Moreover, the United States has repeatedly reserved the right to use nuclear weapons “in extreme circumstances to defend  vital interests, and those of [its] allies.” Russia, France, the United Kingdom and China have issued similar pronouncements. These reservations, at the least, would qualify these states as persistent objectors to any potential emerging customary norm. However, the fact that all of the nuclear power states, with the likely exception of North Korea, could be viewed as persistent objectors defies that categorization. Rather, as a matter of law, this opinio juris stands in the way of a customary norm prohibiting use.
As explained by the ICJ in its Nuclear Weapons advisory opinion, “it is a fact that a number of States adhered to that practice [deterrence] during the greater part of the Cold War and continue to adhere to it. Furthermore, the members of the international community are profoundly divided on the matter of whether non-recourse to nuclear weapons over the past 50 years constitutes the expression of an opinio juris. Under these circumstances the Court does not consider itself able to find that there is such an opinio juris.” It is clear that there is no “general practice accepted as law” that supports any claim there is a comprehensive prohibition on the use of nuclear weapons.
While the fiftieth state’s ratification of the TPNW is certainly a momentous occasion for those who support the admirable goal of eradicating nuclear weapons, the moment is, unfortunately, more symbolic than it is indicative of a customary proscription on their use. The treaty will enter into force. However, its legal effect on nuclear powers is much ado about nothing, for no nuclear weapon state has ratified the TPNW, nor is one likely to do so. And despite consistent and long-standing abstention from the use of nuclear weapons, a customary rule of international law has not emerged prohibiting their use.
It merits emphasis, however, that any employment of nuclear weapons would still have to comply with the law on the use of force—the jus ad bellum—pursuant to Articles 2(4) and 51 of the U.N. Charter and their customary international law counterparts. That includes the requirements of necessity and proportionality. Furthermore, as is the case for all weapons, actual use is also subject to the law of armed conflict rules that govern the conduct of hostilities, such as distinction, proportionality and the requirement to take precautions in attack. The effect of the application of these rules is that the use of nuclear weapons would be lawful only in limited operational contexts, such as a potential nuclear strike using a low-yield weapon on a tank formation in an isolated desert or enemy submarine fleet in the middle of the ocean. In any case, the analysis would depend on the nature of the threat, the value of destroying the objective, the character, size and likely effects of the weapon, and the magnitude of risk to civilians.
Editor’s note: Readers may also be interested in these recent articles at Just Security:
Lawrence Korb, How I Came to Support the Treaty Prohibiting Nuclear Weapons, November 19, 2020|
Daryl G. Kimball, A Turning Point in the Struggle Against the Bomb: The Nuclear Ban Treaty Ready to Go Into Effect, October 27, 2020