It is restricting the spread of nuclear technology to nations beyond by those nations who already have the technology, so that the technology is not used as a weapon of threat by nations that are considered to be having no responsibiliity in its political system.
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Nuclear proliferation is a term now used to describe the spread of nuclear weapons, fissile material, and weapons-applicable nuclear technology and information, to nations which are not recognized as "nuclear weapon States" by the Treaty on the Nonproliferation of Nuclear Weapons, also known as the Nuclear Nonproliferation Treaty or NPT. At present, 189 countries are States Parties to the Treaty on the Nonproliferation of Nuclear Weapons, more commonly known as the Nuclear Nonproliferation Treaty or NPT. These include the five Nuclear Weapons States recognized by the NPT: the People's Republic of China, France, Russian Federation, the UK, and the USA.