80 Years Since Hiroshima and the Development of International Law: Governing the Use of Nuclear Weapons

August 6, 2025, marks 80 years since the United States became the first, and last, state to use nuclear weapons in offensive combat. In June 2024, the U.S. and her European allies celebrated the octogintennial anniversary of the invasion of Normandy, which set the conditions to end the war in Europe in less than a year. Marking eight decades since the US, by itself, but with the acquiescence of the UK under the Quebec Agreement, employed the most horrific weapon of war in history, should be a more somber affair. Combat against Axis forces in Europe was vicious and cruel, but the devastation wreaked using atomic weapons in Japan was categorically different.

The employment of the Little Boy on Hiroshima on August 6, and Fat Man on Nagasaki three days later, signaled the beginning of a new era of warfare that has loomed over every major state-on-state conflict and the conduct of international relations since 1945. The anniversary marks an opportunity to revisit the permissibility of using nuclear weapons under international law, a question made salient by the US use of these weapons on urban areas hosting Japanese military facilities. The weapons killed about 240,000 people in the initial explosions and months-long aftermath, over ninety percent of whom were likely civilians. Hiroshima and Nagasaki set the stage for development of nuclear weapons of increasing yield and lethality, raising questions of whether it is even possible to employ nuclear weapons in compliance with the international law principles of distinction, proportionality, military necessity, and humanity.

Only one quasi-authoritative source on this issue exists – the 1996 Advisory Opinion issued by the International Court of Justice. “Quasi-authoritative” is an appropriate modifier for several reasons, related to the Court’s limited power, the scope of the inquiry, and the caveats in the opinion. Even so, the Advisory Opinion represents the best assessment of the international legal issues surrounding the use of nuclear weapons, which serves as a baseline for interstate negotiation, relations and intercourse on issues regarding development, counterproliferation, platform basing and employment. The International Court of Justice’s bedrock foundation in the UN Charter gives it virtually ultimate authority over issues of international law.

The International Court of Justice (ICJ). The ICJ is a United Nations organ established under Chapter XIV of the UN Charter in 1945. It is empowered by the Charter to decide cases between states, and to issue advisory opinions, which are expositions of the state of international law on an issue without a case between states underlying it. The details regarding its composition, powers, and functions are set out in the Statute of the International Court of Justice, authorized by the UN Charter at Article 92 and annexed to the UN Charter. The Court is authorized to hear any case referred to it by the parties or issues committed to its jurisdiction by treaty. The Court hears cases regarding the interpretation of treaties, questions of international law, situations that would constitute a breach of an international obligation, or reparations germane to the breach of an international obligation. The enforcement mechanism for decisions and orders of the Court is primarily voluntary – Article 94 of the UN Charter commits state parties to comply with the Court’s decisions.  Under Articles 96 of the Charter and 65 of the Statute, the Court may give advisory opinions – legal opinions not related to an actual case – “on any legal question at the request of whatever body may be authorized by or in accordance with the Charter…to make such a request.”

The Advisory Opinion.  In the Nuclear Weapons Advisory Opinion case, the matter was referred to the Court by the United Nations General Assembly, through Resolution 49/75K of December 15, 1994. The vote requesting the opinion was hardly overwhelming but represented a minority of the member states: 78 states voted in favor of the resolution requesting the opinion, while 43 voted against it. 38 states affirmatively abstained, and 26 did not vote at all. The question certified to the court was “Is the threat or use of nuclear weapons in any circumstance permitted under international law?” After accepting the input of 42 states through written pleadings, including four of the five declared nuclear weapon states (UK, France, UK, and the USSR), but not including the People’s Republic of China, the Court delivered its opinion.

The Court first considered the legality of the possession of nuclear weapons, as opposed to use, presumably on the theory that if possession is unlawful, the Court would not have to reach the question of use.  The Court noted Paragraphs 37-50 of the UN Charter addressing the use of force between states is silent on the types or categories of weapons, nothing “[t]he Charter neither expressly prohibits, nor permits, the use of any specific weapon, including nuclear weapons.”  The Court rejected the analogy of nuclear weapons to poison, bacteriological or chemical weapons, implying stretching these categorizations to include the unique nature of nuclear weapons would render the categories meaningless.  Finally, despite the absence of history of use since 1945 and a range of resolutions opposing their use, the Court could not find a norm of customary international law forbidding their possession.

Of course, all usual international law norms applying to the use of force between states apply to nuclear weapons as well. The same legal standard applies to conventional or nuclear weapons, but in terms of jus ad bellum (self defense under Article 51 or pursuant to a UN Security Council resolution under Chapter VII of the UN Charter) and jus in bello – applying the usual principles of discrimination, proportionality, humanity, and military necessity. The court came close to rendering a blanket opinion making the use of nuclear weapons in armed conflict unlawful across the board, noting “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” given how difficult it would be to honor the principles of distinction and proportionality when using nuclear weapons.  For example, while the use of a nuclear weapon against a military weapons depot or production facility outside a major city might comply with the principle of military necessity, it is almost impossible to give sufficient weight in such a circumstance to the principle of distinction, in that tens of thousands of civilians would necessarily be killed in such a strike.

Rather than issuing a blanket rule, the Court left open the possibility of lawful use of nuclear weapons “in an extreme circumstance of  , in which the very survival of a State would be at stake.” Technically, the Court did not opine on this issue, noting it could not “conclude definitively whether the threat or use of nuclear weapons would be lawful or unlawful” in such a circumstance.  On this issue, there were seven votes for this proposition and seven votes against, insufficient to make a declaration one way or the other, requiring the Court essentially to hang the question, neither resolving it in the affirmative or the negative. Finally, though not necessary to the resolution of the opinion, the Court noted “there exists an obligation to pursue in good faith and bring to a conclusion negotiations leading to nuclear disarmament” for all nations.

In short, faced with an opportunity to resolve the issue with finality, even by a thin margin, the Court failed to reach a conclusion.  Even so, by issuing the opinion, the Court provided states with more foundation than existed prior to the Court’s action, and recentered the baseline for negotiations and discussions between states on this key issue regarding employment of nuclear weapons.  In the intervening almost three decades since 1996, momentum has gathered against the possession and use of nuclear weapons more than ever before.  For example, in 2017, the UN General Assembly adopted the Treaty on the Prohibition of Nuclear Weapons, which entered into force on January 22, 2021. The treaty’s adoption led to the International Campaign to Abolish Nuclear Weapons (ICAN) receipt of the 2017 Nobel Peace Prize.  The treaty has been signed by 94 states, with 73 of them becoming state parties. None of the signatories or parties are nuclear powers or NATO member states, greatly diminishing the treaty’s power under international law. Still, it represents movement on the issue and raises the specter that if the 1996 Advisory Opinion were issued today, a different result might obtain, based on the antinuclear momentum of the intervening decades.  To conclude, the 80th anniversary of the nuclear attacks on Hiroshima and Nagasaki are an excellent occasion to reflect on whether the most horrible weapons imaginable, made vastly more lethal and efficient at killing since 1945 while remaining unchanging in their ability to distinguish between military and civilian targets, could ever be used in a manner consistent with the law of nations. This probable inability to comply should affect the way nations see these weapons, their maintenance, and the constraints on their proliferation and employment into the future.