Let's begin by talking about the history and goals of the Treaty on the Non-Proliferation of Nuclear Weapons (NPT). The NPT officially went into effect on March 5, 1970 with the triple goal of disarmament of nuclear weapons for the five Nuclear-Weapons States (those that had detonated a nuclear device...
Let's begin by talking about the history and goals of the Treaty on the Non-Proliferation of Nuclear Weapons (NPT). The NPT officially went into effect on March 5, 1970 with the triple goal of disarmament of nuclear weapons for the five Nuclear-Weapons States (those that had detonated a nuclear device before 1967, namely, the US, Russia, China, France, and the United Kingdom); nonproliferation of nuclear weapons among the Non-Nuclear-Weapons States; and the guarantee of the use of nuclear technology for peaceful purposes.
Currently, 191 states abide by the treaty (or at least claim to). India, Pakistan, Israel, and South Sudan will not sign the treaty, and North Korea has withdrawn. The implementation of and compliance with NPT is reviewed every five years with a Review Conference, and four Preparatory Committee meetings happen in between Review Conferences. These conferences and meetings explore how states have or have not been complying with the NPT, and the Review Conferences issue resolutions and action plans to help enforce the treaty.
Member states are legally bound by the treaty, and it should greatly affect the political decision making of nuclear powers, yet the treaty is only as strong as the commitment of its members. Several Non-Nuclear-Weapons States, for instance, have indeed developed nuclear weapons in spite of their commitment to the treaty. These states may face economic sanctions and a loss of status in the international community, but enforcement falls upon individual countries. The NPT itself cannot enforce its own provisions. It depends on the integrity of its member countries. What's more, a country can freely withdraw from the treaty with only a three month notice. It is then no longer bound to observe the treaty. This has already happened with North Korea.
Basically, then, while the treaty certainly does affect decisions within its member countries as they strive to control nuclear weapons and still make good use of nuclear technology, the treaty lacks the power of enforcement for countries that violate it.