The terms of the agreement, enshrined in an exchange of letters between Mrs. Thatcher and President Carter, are almost identical to those of the 1962 Nassau Agreement, which preceded the Polaris sales contract. 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.  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. 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 2007, a disagreement broke out between mod and the Ministry of Finance over the financing of the cost of capital of the succession program. The MOD indicated that the cost of capital for the acquisition of nuclear deterrence had been borne in the past by the Ministry of Finance, a position which the Ministry of Finance refuted. The argument focused on an increase in the defence budget announced as part of the 2007 Comprehensive Expenditure Review (CSR). The CSR regime proposed that the increase allow the MOD to « take steps to maintain nuclear deterrence, » which some commentators saw as an obligation to finance the cost of the project`s capital. However, the MOD confirmed in November 2007 that, although additional resources had been allocated to the mod budget, the program`s expenditures would come from the core equipment budget.