Preventing Proliferation Nuclear proliferation became a major concern in the 1960s as more nations developed the capability to build bombs. The Non-Proliferation Treaty (NPT) was negotiated by a United Nations committee and opened for signatures in 1968. It became legally binding on participants in 1970. Under the treaty, countries that built bombs before 1967—the United States, the former Soviet Union (now Russia), the United Kingdom, France, and China—are classified as nuclear-weapon states and may continue to possess them. Other signatory nations pledge never to build nuclear weapons but may benefit from peaceful development of nuclear energy with assistance from nuclear-weapon states. As of January 2021, 191 states agreed to honor the treaty. Above: First meeting of the Preparatory Committee for the Review Conference of the Parties to the NPT, United Nations, Geneva, Switzerland, April 1, 1974. Photograph courtesy of UN Photo. Left: The review and extension meeting, where an indefinite extension of the treaty was agreed to in May 1995. Article VI further committed the signatories to pursuing an end to the nuclear arms race, eliminating nuclear weapons, and ultimately achieving complete international disarmament. Photograph by Jon Levy/AFP/Getty Images.