Most States seek nuclear weapons for their deterrent qualities, often pursuing them because they fear that their neighbours are developing such weapons. Nuclear-weapon-free zones are intended to fix this security dilemma because they prohibit the possession, testing, transporting and stationing of nuclear weapons within a specific area. Without the presence of nuclear material in a region, no country should feel insecure enough to seek or develop such weapons. At its foundation, NWFZs are confidence-building measures aimed at improving trust and transparency among neighbouring countries.
In fact, the oldest NWFZ predates the NPT (Non-Proliferation Treaty) by a year. It was established in 1967 in Latin America and the Caribbean. In 1968, the NPT explicitly granted regional blocs the right to create NWFZs: “Nothing in this Treaty affects the right of any group of States to conclude regional treaties in order to assure the total absence of nuclear weapons in their respective territories,” (Article VII, NPT). Since then NWFZs have been successfully established in the South Pacific and South East Asia. NWFZs have also been created, though they are not yet in force, in Africa and Central Asia. More recently, post-Cold War Mongolia declared itself to be a single-state NWFZ and the international community has accepted this designation. Surrounded by Russia and China you could hardly expect it to create a regional zone.