American-made submarine-launched ballistic missile (SLBM) that succeeded the Poseidon and Polaris missiles in the 1980s and '90s. Under development from the late 1960s, the Trident developed into two models. The Trident I, or C-4, is 34 feet (10.4 m) long and 6 feet (1.8 m) in diameter. It can deliver eight independently targetable 100-kiloton nuclear warheads to a range of 4,600 miles (7,400 km). The Trident II, or D-5, is about 46 feet (14 m) long and carries an average of 10 475-kiloton warheads. It has a maximum range of 7,000 miles (11,300 km).

The Trident warheads are launched by three solid-fueled booster stages and are dispersed toward their targets by a liquid-fueled “bus” in the missile's front end. With inertial guidance refined by stellar or satellite navigation, Tridents are more accurate than most land-based ballistic missiles. Their accuracy gave them the ability, unprecedented among SLBMs, to threaten hardened missile silos and command bunkers in the Soviet Union, and their extended range allowed their submarines to patrol almost anywhere in the Atlantic and Pacific oceans, making detection extremely difficult.

Beginning in 1979, Trident I missiles were fitted aboard older Poseidon-carrying submarines and newer Ohio-class vessels, which were built with larger missile tubes designed to accommodate the Trident II in the 1990s. In 1986 the United Kingdom began to build new Vanguard submarines to carry the Trident II, which was to have warheads of British design.