<Tr> <Td_colspan="2"> Nuclear Non-Proliferation Treaty at Wikisource </Td> </Tr> <P> The Treaty on the Non-Proliferation of Nuclear Weapons, commonly known as the Non-Proliferation Treaty or NPT, is an international treaty whose objective is to prevent the spread of nuclear weapons and weapons technology, to promote cooperation in the peaceful uses of nuclear energy, and to further the goal of achieving nuclear disarmament and general and complete disarmament . </P> <P> Opened for signature in 1968, the treaty entered into force in 1970 . As required by the text, after twenty - five years, NPT Parties met in May 1995 and agreed to extend the treaty indefinitely . More countries have adhered to the NPT than any other arms limitation and disarmament agreement, a testament to the treaty's significance . As of August 2016, 191 states have adhered to the treaty, though North Korea, which acceded in 1985 but never came into compliance, announced its withdrawal from the NPT in 2003, following detonation of nuclear devices in violation of core obligations . Four UN member states have never accepted the NPT, three of which are thought to possess nuclear weapons: India, Israel, and Pakistan . In addition, South Sudan, founded in 2011, has not joined . </P> <P> The treaty defines nuclear - weapon states as those that have built and tested a nuclear explosive device before 1 January 1967; these are the United States, Russia, the United Kingdom, France, and China . Four other states are known or believed to possess nuclear weapons: India, Pakistan, and North Korea have openly tested and declared that they possess nuclear weapons, while Israel is deliberately ambiguous regarding its nuclear weapons status . </P>

When did the nuclear non proliferation treaty start