International law on nuclear weapons is complex, reflecting a deep divide between nuclear-weapon states (NWS) and non-nuclear-weapon states (NNWS). No single treaty comprehensively prohibits the use of nuclear weapons for all states; rather, the issue is addressed through multiple instruments, treaties, and customary law principles.

The Treaty on the Non-Proliferation of Nuclear Weapons (NPT), 1968, is the cornerstone. It aims to prevent nuclear weapons proliferation, promote disarmament, and encourage peaceful uses of nuclear energy. The NPT establishes a two-tier system: NWS (the P5) commit to disarmament, while NNWS agree not to acquire nuclear weapons. This implicitly permits the P5 to retain nuclear weapons, making the disarmament pillar a frequent point of contention. The International Court of Justice (ICJ), in its 1996 Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons, could not definitively declare the threat or use unlawful in all circumstances, highlighting continuing legal ambiguity and tension within international law.

The Treaty on the Prohibition of Nuclear Weapons (TPNW), 2017, represents a significant paradigm shift. It is the first legally binding agreement comprehensively prohibiting nuclear weapons, aiming at their total elimination. It prohibits developing, testing, producing, manufacturing, possessing, stockpiling, transferring, using, or threatening use of nuclear weapons, as well as assisting, encouraging, or hosting them on its territory.

Critically, TPNW emerged from humanitarian concerns about catastrophic consequences. However, its practical effectiveness is limited: no NWS, NATO member, or nuclear umbrella state has signed or ratified it, arguing it undermines the NPT and the concept of nuclear deterrence. While legally and morally significant for parties, it currently has little impact on existing arsenals or policies. The treaty highlights the fragmented state of international law on nuclear weapons, offering a pathway to a nuclear-weapon-free world, but its long-term impact depends on building normative pressure and eventual inclusion of NWS.