At a time when military tensions in the Middle East have further intensified in recent weeks, in the context of operations conducted by the United States and Israel and renewed concerns surrounding Iran’s nuclear programme, a recurring question has re-emerged in public debate: why does Israel possess nuclear weapons while Iran is legally prohibited from acquiring them?Often framed in terms of unequal treatment or “double standards,” this question actually relates to the very structure of the international legal regime governing nuclear weapons.International law is a legal order created by states and for states. As such, it is grounded in the consent of states, which derives from their sovereignty. This fundamental principle also applies to the legal regime governing nuclear weapons: the possession of nuclear weapons – or the decision to renounce them – is a matter of sovereign choice. In other words, only a state may consent to limiting its military capabilities by renouncing possession of such weapons of mass destruction.This state voluntarism is clearly reflected in the 1968 Treaty on the Non-Proliferation of Nuclear Weapons (NPT).What’s the NPT?This treaty constitutes one of the pillars of collective security in international law. Its purpose is to prevent the proliferation of nuclear weapons – in particular to additional states – in order to promote nuclear disarmament, and encourage safe and peaceful use of nuclear energy.The NPT distinguishes between nuclear-weapon states (United States, Russia, the United Kingdom, France, and China) and non-nuclear-weapon states. More specifically, it defines nuclear-weapon states as those that had manufactured and exploded a nuclear weapon or other nuclear explosive device before 1 January 1967, while the other parties are classified as non-nuclear-weapon states and have agreed to forswear the possession of nuclear weapons.Nuclear power: different rules for the haves and the have notsThis fundamental distinction structures the entire legal regime of the treaty and entails differentiated but complementary obligations among the 191 States parties. While non-nuclear-weapon states agree not to acquire nuclear weapons, nuclear-weapon states agree not to transfer such weapons or assist other states in acquiring them. The treaty also stipulates an obligation to pursue negotiations in good faith towards nuclear disarmament.In a context marked by Cold War tensions and fears of an uncontrolled proliferation of nuclear powers, the spread of nuclear weapons to an increasing number of states was perceived in 1968 as a major factor of international instability and a heightened risk of nuclear conflict. Against this backdrop, the compromise at the heart of the NPT was accepted, based on a differentiated allocation of obligations between nuclear-weapon states and non-nuclear-weapon states.Although this asymmetrical compromise may, at first glance, appear unequal, it was designed as an instrument of strategic stability and collective security, while also constituting a direct expression of state sovereignty. A state may indeed decide to limit its own prerogatives. In return, breaches of these commitments entail legal consequences. This is where the debate over the different treatment of Iran and Israel arises.If Israel has a nuclear arsenal, why can’t Iran?Iran has been a party to the NPT since 1970 and is legally bound, as a non-nuclear-weapon state, not to acquire nuclear weapons, while also being subject to the safeguards mechanisms of the International Atomic Energy Agency (IAEA). Its nuclear programme is therefore assessed within a treaty framework that imposes specific legal obligations and international verification requirements.By contrast, Israel is not a party to the NPT. Under the principle of the relative effect of treaties, Israel, not being a party to the NPT, cannot be legally bound by obligations arising from that treaty.A legal framework producing differentiated outcomesThe difference in treatment between the two countries thus stems less from a legal inconsistency than from the very logic of international law. It illustrates the coexistence within the international order of de jure nuclear-weapon states and de facto nuclear-weapon states, such as Israel.Several states currently possess nuclear weapons outside the framework of the NPT. In addition to Israel, this is also the case for India, Pakistan and North Korea (since its withdrawal from the treaty in 2003). Their situation does not, in itself, constitute a violation of the NPT, since they are not (or are no longer) parties to it. They therefore operate within a legal framework distinct from that applicable to states bound by the treaty.This situation reflects a fundamental feature of the international legal order: the coexistence of treaty regimes to which not all states necessarily adhere. One example is the 2017 Treaty on the Prohibition of Nuclear Weapons (TPNW), to which 74 states are currently parties, and which prohibits the acquisition, possession and use of such weapons. This legal regime coexists alongside that of the NPT.Accordingly, the question of why Israel possesses nuclear weapons, while Iran is denied that possibility is less a reflection of a contradiction in international law than a consequence of its structure.In the absence of a treaty commitment, general international law does not currently establish a comprehensive and absolute prohibition on the possession of nuclear weapons as such. Only those states that have consented to specific obligations – notably within the framework of the NPT or the TPNW – are legally bound.This analysis is supported by the jurisprudence of the International Court of Justice. In its 1986 judgement in Military and Paramilitary Activities in and against Nicaragua, the Court stated:“In international law there are no rules, other than such rules as may be accepted by the state concerned, by treaty or otherwise, whereby the level of armaments of a sovereign state can be limited, and this principle is valid for all states without exception.”Thus, in accordance with the principle of sovereign equality, limitations on military capabilities derive solely from state consent. In the absence of such a commitment, no state may legally impose on another an obligation of renunciation.International law governing nuclear weapons therefore does not establish any general right to possess such weapons.Rather, it reflects the existence of sovereign commitments through which some states have chosen to renounce them, while others have decided not to subject themselves to such constraints.This article was co-authored with Débora Surreco Carrilho, PhD candidate in international law, University of Orléans (France). A weekly e-mail in English featuring expertise from scholars and researchers. It provides an introduction to the diversity of research coming out of the continent and considers some of the key issues facing European countries. 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