To date, the world has witnessed five armed attacks on states' nuclear programs: the U.S. response to the Cuban missile crisis (1962), Israel's attack on Iraq’s Osirak reactor (1981), the coalition states’ military strike on Iraq’s weapons of mass destruction program (2003), Israel’s attack on the Al-Kibar in Syria (2007), and most recently, the June 2025 Israeli attack—carried out in cooperation with the United States—on Iran’s nuclear facilities. Do these precedents reflect the emergence of a new exception to the prohibition of the use of force? That is, can the possession of a military nuclear program by a non-nuclear-weapon state, due to the serious threat it poses to international peace and security, justify a preemptive attack aimed at destroying or halting that program? This article examines existing state practice and, in particular, the Advisory Opinion of the ICJ on the Legality of the Threat or Use of Nuclear Weapons. It concludes that although the formerly consistent state practice rejecting such attacks has fragmented, the current divergence among states does not yet permit the emergence of a new exception to the peremptory norm of the prohibition of the use of force

H.L.A. Hart, the renowned American lawyer, believed that those legal rules which “restrict the free use of violence” constitute the inherent rules of any legal order. On this basis, some theorists argue that, since international law has only gravitated toward such rules in its more recent development in the twentieth century, for much of its existence it could not truly be regarded as a legal system. The reality is quite different. In fact, international law, from ancient Greece to the present day, has contained rules aimed at limiting violence in international relations. Article 2(4) of the UN Charter, which prohibits all forms of the use of force, represents the most comprehensive of such rules. Although this provision explicitly prohibited “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 UN”, a number of events have generated persistent challenges and ambiguities regarding the scope of this prohibition. The most recent of these events were the attacks by Israel and the U.S. against Iran’s nuclear and missile facilities during operations “Rising Lion” and “Midnight Hammer” in June 2025. As this article will demonstrate, Israel and the U.S. sought to justify their actions by invoking a spectrum of arguments, including the alleged existential threat posed by Iran’s nuclear program. This incident, standing alone, might not suffice to reshape the scope of the prohibition of the use of force; however, when considered alongside other precedents, the issue becomes more complex.

Indeed, prior to the recent strikes against Iran, there have been other instances of the use of force against the nuclear program of “non-nuclear-weapon States.” According to Article 9 of the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), States parties that had not conducted a nuclear explosive test before January 1967 are classified as non-nuclear-weapon States and are obligated not to receive or manufacture nuclear weapons. In the past half-century, armed operations have been employed five times against such States accused of breaching their nuclear obligations: the U.S. response to the transfer of nuclear missiles to Cuba (1962); Israel’s attack on the Osirak reactor in Iraq (1981); the U.S.-led invasion of Iraq with more than 30 other States (2003); Israel’s strike on Syria’s Al-Kibar reactor (2007); and, most recently, the Israeli and U.S. attacks against Iran (2025). Given that the methodology in international law often revolves around State practice , these precedents raise a fundamental question: Has a rule emerged that permits the use of force against non-nuclear-weapon States which is accused of breaching their nuclear obligations?

This article seeks to address this question in four parts. The first section examines the nature of the prohibition of the use of force and its exceptions. Considering the word-limit and the fact that the nature of this prohibition and its exceptions have already been extensively studied elsewhere , this section will be concise and avoid delving into theoretical and practical controversies. The second section offers a factual analysis of the five episodes of the use of force against non-nuclear-weapon States, accompanied where appropriate by legal assessments. A more detailed and legal examination, however, is reserved for the third section, which explores the impact of these precedents on the structure of international law, particularly in light of the formation of customary rules, the law of State responsibility, and the jurisprudence of the International Court of Justice (Court or ICJ).

International law has, to date, observed five instances of State practice regarding the use of force in response to the violation of nuclear obligations by non-nuclear-weapon States. This article has demonstrated that, at present, the mere development of nuclear weapons, by itself and without integration with other factors, does not reach the threshold of a threat that would justify the use of force, even under the controversial theory of anticipatory self-defense. Furthermore, significant divergence among States and within the doctrine prevents these practices from constituting settled customary law. Nevertheless, because the methodology of international law revolves around State practice, particularly the silence or inaction of some States regarding the two most recent cases—the strike on Syria’s Al-Kibar reactor and the attacks on Iran’s nuclear facilities— may be seen as initial indicators of a potential shift in the rules governing the use of force.

Although this new perspective constitutes an international law violation under current norms, the emergence of certain new rules inevitably occurs through the breach of current norms. According to this new perspective, the potentially catastrophic power of nuclear weapons, when combined with certain factual circumstances, including the political stance of a State which seeks to acquire those weapons, constitutes a threat that authorizes the threatened State or the international community to resort to force to neutralize it. In other words, since a nuclear strike leaves no alternative for the threatened or attacked State to respond proportionally, the exceptions to the prohibition of the use of force should, compared with conventional threats, be interpreted less restrictively to allow the threatened State to counter the threat. Whether international law will, in its further development, embrace a new exception to the prohibition of the use of force, or continue to regard it as a violation of its norms, remains to be seen through time and subsequent practice.