The LGM-118 Peacekeeper, originally known as the MX for "Missile, Experimental", was a MIRV-capable intercontinental ballistic missile (ICBM) produced and deployed by the United States from 1985 to 2005. The missile could carry up to twelve Mark 21 reentry vehicles (although treaties limited its actual payload to 10), each armed with a 300-kiloton W87 warhead. Initial plans called for building and deploying 100 MX ICBMs, but budgetary concerns limited the final procurement; only 50 entered service. Disarmament treaties signed after the Peacekeeper's development led to its withdrawal from service in 2005.
Studies on the underlying concept started in the 1960s. The idea was to allow the U.S. to absorb a sneak attack by the USSR with enough warheads surviving to attack the remaining Soviet missile silos. To do so, the missiles had to be highly accurate, be based in such a way that enough would survive a nuclear attack, carry a large number of warheads so the survivors would still inflict massive damage, and be able to rapidly re-target so they could be aimed at only those missiles the Soviets had not yet fired. Ultimately, the only problem that was never completely solved was the issue of basing; over its development through the 1970s, MX emerged as a highly accurate, rapid-firing, and quickly retargeted system.
Initial development began in 1971, with full-scale development starting in 1974. President Jimmy Carter ordered initial production in 1979 but was overturned by Congress. After considerable debate about the system, in October 1982, President Ronald Reagan announced that 50 of the newly named Peacekeepers would be put into service in existing LGM-30 Minuteman silos, a temporary measure until final basing was decided. The first flight test took place in 1983, which included the successful launch of six inert re-entry vehicles, each hitting pre-planned targets. It was the first U.S. ICBM to use a cold launch system, and it reached initial operational capability in 1986.