The Nuclear Non-Proliferation Treaty (1968)
This was signed by the USA, USSR, Britain and 59 other states.
It was divided into two categories:
Nuclear weapon states (NWS) such as the USA, USSR, China, France and Britain,
Non Nuclear weapon states (NNWS)
It stated that NWS would work towards disarmament and play no part in helping other nations to develop nuclear weapons
NNWS at the time agreed not to develop nuclear weapons, mostly because at this time they did not have the financial or technological abilities to do that anyway.