What treaty was signed in 1968 between nuclear powers?

The Treaty on the Non-Proliferation of Nuclear Weapons, also called the Nuclear Non-Proliferation Treaty, was an agreement signed on July 1, 1968, by the United Kingdom, the United States, the Soviet Union, and 59 other states, under which the three major signatories, which possessed nuclear weapons, agreed not to assist other states in obtaining or producing them. The treaty became effective in March 1970 and was to remain so for 25 years.

Source: Britannica