An international agreement intended to prevent the spread of nuclear technology. It was signed by the U.S., Britain, the Soviet Union, and 59 other countries in 1968. The three major signatories agreed not to assist states lacking nuclear weapons to obtain or produce them; the non-nuclear signatories agreed not to attempt to obtain nuclear weapons in exchange for assistance in developing nuclear power for peaceful purposes. France and China, both nuclear powers, declined to ratify the treaty until 1992, and some nuclear powers, including Israel and Pakistan, have never signed it. In 1995, when the treaty was due to expire, it was extended indefinitely by a consensus vote of 174 countries at the United Nations.