The Trident missile is a submarine-launched ballistic missile (SLBM) equipped with multiple independently targetable reentry vehicles (MIRV). Originally developed by Lockheed Missiles and Space Corporation, the missile is armed with thermonuclear warheads and is launched from nuclear-powered ballistic missile submarines (SSBNs). Trident missiles are carried by fourteen US Navy Ohio-class submarines, with US warheads, and four Royal Navy Vanguard-class submarines, with British warheads. The missile is named after the mythological trident of Neptune.
In 1971, The US Navy began studies of an advanced Undersea Long-range Missile System (ULMS). A Decision Coordinating Paper (DCP) for the ULMS was approved on 14 September 1971. ULMS program outlined a long-term modernization plan, which proposed the development of a longer-range missile termed ULMS II, which was to achieve twice the range of the existing Poseidon (ULMS I) missile. In addition to a longer-range missile, a larger submarine (Ohio-class) was proposed to replace the James Madison and Ben Franklin class SSBNs in 1978. The ULMS II missile system was designed to be retrofitted to the existing SSBNs, while also being fitted to the proposed Ohio-class submarine.