In their history as a Royal Navy submariner, Peter Hennessy and James Jinks describe this turbulent story as a “nuclear neuralgia of Labour.” What their leaders wanted was often difficult to know. The manifesto for the 1964 elections stated that the Nassau agreement to purchase Polaris nuclear missiles in the United States “would not add anything to the deterrent force of the Western alliance… [Polaris] won`t be independent, and he won`t be British and he won`t be afraid… We are no longer prepared to waste the country`s resources with endless duplication of strategic nuclear weapons. Nothing could be clearer. The military establishment, including the then Chief of the Defence Staff, Lord Mountbatten, firmly believed that polaris would be nullified if the Labour Party won; the government encouraged the Admiralty to spend as much as possible on the submarine program to make cancellation more difficult. But the Labour Party`s victory when it arrived changed little, apart from the planned reduction of the polaris fleet from five to four submarines. The Trident missile is a submarine-launched ballistic missile (SLBM) equipped with several independent re-entry vehicles (MIRVs). Originally developed by Lockheed Missiles and Space Corporation, the rocket is armed with thermonuclear warheads and is launched by nuclear submarines (SSBNs). The Trident missiles are carried by fourteen United States Navy Ohio-class submarines equipped with American warheads and four Royal Navy Vanguard-class submarines equipped with British warheads. The rocket is named after Neptune`s mythological trident.  In January 1979, Callaghan addressed President Jimmy Carter, who reacted positively but without commitment.  The Carter administration`s main priority was the SALT II agreement with the Soviet Union, which limited the stockpile of nuclear weapons. It was signed in June 1979, but Carter faced a fierce fight for its ratification by the United States Senate.
 MIRV technology proved to be a major flaw in the 1972 SALT-I agreement, which had a limited number of missiles but no warheads. During the SALT II negotiations, the United States had opposed Soviet proposals to include British and French nuclear forces in the agreement, but the concern was that the supply of MIRV technology to the United Kingdom would be considered by the Soviets as a violation of the spirit of the SALT II non-circumvention clause.  Negotiations began on 8 February, with the British team again led by Wade-Gery. The Americans expressed concern about the proposal to reduce British defence and insisted that the aircraft carrier HMS Invincible remain in service, which they considered necessary to avoid the problems associated with the Belize-Guatemalan territorial dispute. They accepted a counter-offer that Britain retain the two landing platform ships HMS Fearless and Intrepid, for which the Americans reduced the R `D fee.  As part of the agreement, the United Kingdom would purchase 65 Trident II D-5 missiles to be operated as part of a common pool of weapons at the Navy Kings naval base in the United States. The United States would maintain and support the missiles, while the United Kingdom would manufacture its own submarines and warheads to mount on the missiles.  Warheads and missiles would be mated in the United Kingdom. This is expected to save about US$500 million over eight years in Coulport, while the Americans have spent $70 million to upgrade the Kings Bay facility. The sales contract was signed on 19 October 1982 by the British Ambassador to the United States, Sir Oliver Wright, and US Secretary of State George Shultz.  Trident missiles are made available to the United Kingdom in accordance with the Polaris purchase agreement amended for Trident in 1963.
 British Prime Minister Margaret Thatcher wrote to President Carter on 10 July 1980 asking him to authorize the delivery of Trident-I missiles.