Trident missiles were made available to the United Kingdom under 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. In 1982, however, Thatcher wrote to President Reagan asking the United Kingdom to acquire the Trident II system, which had been accelerated by the U.S. Navy. This was agreed in March 1982.  As part of the agreement, the United Kingdom still paid 5% of its $2.5 billion in total procurement costs to the U.S. government as a contribution to research and development.  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.  The Trident missile is an underwater ballistic missile (SLBM) equipped with several independent reentry 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.  The law could be repealed if the President stated that it was in the interest of the United States, but the Carter administration wanted to commit to the United Kingdom increasing defence spending by the same amount or paying the occupancy costs of Rapier batteries and Ground Launched (GLCM) cruise missile sites in the United Kingdom. On June 2, 1980, Thatcher and U.S. Secretary of Defense Harold Brown agreed on $2.5 billion for the C-4 missile system, as well as a 5% tax on research and development, British personnel for rap batteries and an extension of the U.S.
base to Diego Garcia, which has become high importance since the Soviet invasion of Afghanistan.   Defence Minister Francis Pym informed the cabinet on 15 July 1980 of the decision to buy Trident and announced it in the House of Commons later that day.  The agreement was amended by an amendment to Polaris` purchase agreement by modifying Polaris to “Trident”.  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.