Distribution (of nuclear weapons). All measures of international nuclear material monitoring serve the non-proliferation of nuclear weapons stipulated in the Treaty on the Non-proliferation of Nuclear Weapons.  188 UN member states and observers, the Holy See, and the State of Palestine are parties to the Treaty.  The treaty came into force and closed for signature on 5 March 1970 with the deposit of ratification of the three depositary states (USA, USSR, UK) and 40 others. Since then, states that did not sign the treaty may only accede to it.