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horizontal proliferation the regime has been relatively successful as at present only nine states possess nuclear weapons (China, France, India, Israel, North Korea, Pakistan, Russia, UK, USA). At the heart of the regime is the nuclear Non-Proliferation Treaty (NPT). Signed in 1968 the NPT recognized the existence of five nuclear weapons states (NWS)—China, France, Russia, UK, USA. In signing, the non-nuclear weapons states (NNWS) committed themselves not to develop nuclear weapons and to make themselves subject to various monitoring procedures. In return for such abstinence the NWS agreed to help the NNWS acquire nuclear capabilities for peaceful purposes (e.g. power production). They also pledged not to use nuclear weapons to attack NNWS unless those states had attacked them while aligned to a nuclear power, and they agreed to pursue their own nuclear disarmament over the longer term.
    Although some states have ignored the NPT and developed nuclear weapons capabilities regardless, this number has been small. Indicative of the emergence and value attached to the non-proliferation regime is that over time various states—including Argentina, Brazil, South Africa, South Korea, and Sweden—abandoned their nuclear weapons programmes. This was also the case with Kazakhstan and Ukraine, which on gaining independence in the 1990s became de facto nuclear powers as a result of the presence of Soviet nuclear weapons installations on their territories. The non-proliferation regime has also been enhanced with the declaration of various Nuclear Weapons Free Zones (NWFZ) in Africa, Latin America, South-East Asia, the South Pacific, and Antarctica, and by the active role of the International Atomic Energy Agency (IAEA), established in 1957, in monitoring and inspecting the use nuclear facilities and verifying that states are upholding their NPT commitments.

2. Current global distribution of nuclear weapons, (2013)
    The non-proliferation regime therefore suggests that states do not always quest after power and may see upholding the regime as operating in their longer-term collective benefit. However, aside from such rational cost–benefit calculations it may also be that NNWS have not pursued nuclear weapons because, owing to their immense and indiscriminate destructive power, their use has become viewed as morally unacceptable to large segments of the international community, except perhaps in retaliation to a first strike by another nuclear power. Acquiring nuclear weapons would therefore be incompatible with the identity many states project to themselves and the world.
    In contrast, progress on vertical proliferation—the NWS’s commitment to pursue complete nuclear disarmament—has been more mixed. From the late 1960s onwards various strategic (nuclear) arms control talks were undertaken and treaties signed. These included restricting the types of nuclear weapons tests which could be undertaken and working towards reducing the overall number of nuclear weapons, although significant progress had to wait until the Cold War’s end. In signing the New Strategic Arms Reduction Treaty in 2010, however, the USA and Russia committed themselves to reducing the number of deployed warheads to 1,550 over seven years. Particularly significant, however, was the signing of the Anti-Ballistic Missile (ABM) Treaty in 1972. This was a treaty specifically designed to ensure the mutual vulnerability of the Soviet Union and the United States by limiting the missile defences they could develop and therefore making them mutually vulnerable to nuclear attack. The ABM Treaty rested on the proposition that the development of effective missile defences by one side would undermine the overall balance of power and might undermine the other side’s confidence in the effectiveness of its nuclear deterrent, incentivizing it to launch a first strike before it was too late. Importantly, in 2002 the USAwithdrew from the Treaty to pursue the development of its