On August 5, 1963, representatives of the United States, the Soviet Union, and the United Kingdom signed the Limited Test Ban Treaty in Moscow. The agreement prohibited nuclear weapons tests in the atmosphere, outer space, and underwater — but permitted them underground — and entered into force on October 10, 1963, with 123 original parties. It was the first major arms control agreement of the Cold War, and its basic bargain has held for more than sixty years.
The LTBT is closely related to, and a precursor of, the SALT framework. See the SALT treaties and arms control.
The U.S., USSR, and UK had been in intermittent negotiations on a comprehensive test ban since 1958. The central obstacle was verification: the U.S. and UK insisted on on-site inspections, which the Soviet Union rejected. The negotiations were formally suspended in early 1962, after the U-2 incident and the resumption of U.S. atmospheric testing. The Cuban Missile Crisis of October 1962 reopened the question. The political climate after the crisis was unusually conducive to compromise — the technical problem of distinguishing small underground tests from conventional explosions had been partly solved.
Kennedy’s American University address of June 10, 1963 called for a reorientation of the U.S.–Soviet relationship; the test ban was the first concrete step. The treaty was negotiated in weeks and signed in Moscow on August 5, 1963.
The LTBT prohibited any nuclear test, or any other nuclear explosion, in the atmosphere, in outer space, and underwater. Underground tests were permitted, as long as radioactive debris did not cross national borders. Verification relied on national technical means — seismic and atmospheric monitoring — and existing diplomatic channels. There was no on-site inspection protocol. The verification of underground testing was a problem that would not be fully addressed until the Comprehensive Test Ban Treaty negotiations of the 1990s.
The treaty’s political significance exceeded its technical scope. Atmospheric testing had become a major public health and political issue by the early 1960s; the Castle Bravo test of March 1954 had spread fallout across the Pacific and onto the Japanese fishing vessel Lucky Dragon. The treaty responded to a real political constituency. France and China, the two nuclear powers that did not sign in 1963, both adhered later: France in 1974, China in 1996. The 1996 Comprehensive Test Ban Treaty, which would have prohibited all nuclear explosions, has not entered into force; the LTBT remains the legally binding global test ban.