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 1986 to 2005. The missile could carry up to eleven Mark 21 reentry vehicles (although treaties limited its actual payload to ten), each armed with a 300-kiloton W87 warhead. Plans called for building and deploying up to 200 MX ICBMs, but budgetary and political 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 US 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. Over its development through the 1970s, MX emerged as a highly accurate, rapid-firing, and quickly retargeted system. Ultimately, the only problem that was never completely solved was the issue of basing.
