The prohibition of nuclear weapons remains one of the most contentious issues in international customary law. Events such as the atomic bombings of Hiroshima and Nagasaki prompted global efforts to limit their use, yet the legal framework governing their use remains deeply contested. Nations in possession of nuclear weapons resist disarmament due to security and sovereignty concerns. 

To determine whether a binding norm exists, it's important to assess whether a prohibition on nuclear weapons has developed under international customary law. This requires an analysis of the law’s core elements, relevant legal instruments, judicial opinions, and regional treaties. 

What Makes a Rule Binding in Customary Law? 
To establish a binding norm under international customary law, two elements must be present:
1. Usus: A consistent and widespread state practice. 
2. Opinio juris: A belief that such practice is legally required. 

Both elements must exist together. States must act out of a sense of legal obligation, not convenience or habit, for a binding norm to emerge. 

Does the UN Charter Set the Standard? 
The UN Charter's primary purpose is to maintain international peace, security, and stability. Although it does not refer to nuclear weapons directly, two provisions are central to the discussion. 

Article 2(4) prohibits the threat or use of force against the territorial integrity or political independence of any state. This principle has attained jus cogens status, making it binding on all states, not only UN members. Article 51, however, preserves the right to collective self-defence in response to armed attack, which nuclear states have used to justify retaining their arsenals as a deterrent.
Because the Charter does not expressly prohibit nuclear weapons, nuclear-armed states argue that no explicit ban exists. They rely on Article 51 to justify responding to perceived threats at their discretion. This legal ambiguity has allowed nuclear deterrence to remain a core part of their defence strategies. 

The NPT: Widely Accepted, but Limited in Scope
The Treaty on the Non-Proliferation of Nuclear Weapons (NPT), in force since 1970, was designed to limit the spread of nuclear weapons and reduce their use. It requires non-nuclear states not to acquire such weapons, while obliging nuclear-armed states to move toward disarmament.

The NPT is widely accepted, strengthening its role as a source of customary international law. However, it is often criticised as unequal and discriminatory, with countries like India refusing to sign on that basis. While the treaty aims to limit the spread of nuclear weapons, it does not ban their use, leaving a clear gap in the legal framework.

The TPNW: Clear Prohibition, Limited Support
The Treaty on the Prohibition of Nuclear Weapons (TPNW), which entered into force in 2021, goes further than the NPT. It imposes uniform obligations on all states not to develop, possess, or assist others in acquiring nuclear weapons. Its explicit aim is the total eradication of nuclear weapons, acknowledging the catastrophic humanitarian consequences of their use.

Despite being adopted by a majority in the UN General Assembly, the TPNW has seen limited uptake. No nuclear-armed state has signed it, and most Security Council members oppose it. 
Without broader acceptance, particularly from states with nuclear capabilities, it cannot be regarded as a rule of customary international law, even if it may meet the theoretical requirements of usus and opinio juris among its signatories. 

The ICJ’s Position: No Clear Customary Rule 
In its 1996 advisory opinion, the International Court of Justice (ICJ) considered whether the threat or use of nuclear weapons is permitted under international law. While it recognised widespread opposition reflected in UN General Assembly resolutions, it also noted consistent objections from nuclear-armed states. 

Applying the doctrine of persistent objection, the Court held that states which consistently reject an emerging norm cannot be bound by it. It concluded that, despite the global concern, it shows insufficient opinio juris to establish a binding prohibition. 

A Strong Dissent: Limits to Self-Defence
Judge Weeramantry issued a notable dissent, arguing that nuclear weapons violate both international law and human rights law, undermining the UN's peace mandate. He contended that even the right to self-defence is not unlimited and must be exercised proportionately. 

He further argued that jus cogens norms, protecting civilians and humanity broadly, cannot be set aside to justify nuclear use, given its catastrophic and indiscriminate consequences.

Regional Action: Africa’s Nuclear-Free Commitment 
The Treaty of Pelindaba, in force since 2009, declared Africa a nuclear-weapon-free zone and was signed by all African Union members, with 44 ratifications, and reflects a strong regional commitment to eliminating nuclear weapons.

However, nuclear-armed states dispute its broader legal impact. While it creates binding obligations within Africa, it does not, on its own, establish a universal rule under customary international law.
 
Conclusion: A Clear Gap in the Law
Currently, no binding customary international law norm prohibiting the use or threat of nuclear weapons has been established. Nuclear-armed states have persistently objected to restrictions, preventing the formation of opinio juris at a global level. 

While regional treaties and majority UN resolutions demonstrate strong opposition to nuclear weapons, they have not translated into enforceable universal obligations.
This gap raises deeper questions about inequality in the international system and the need for a more consistent legal framework, particularly regarding weapons whose consequences extend far beyond the states directly involved.

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