LGM-118 Peacekeeper (MX)

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The LGM-118 Peacekeeper (MX) was a solid-fueled intercontinental ballistic missile built by the United States designed to attack hardened military targets.

Peacekeeper at a Glance

Originated From
United States
Intercontinental Ballistic Missile (ICBM)
Possessed By
United States
21.1 m
2.34 m
Launch Weight
87,750 kg
10 Mk 21 RVs on MIRV Platform
W-87, 300-475 kT Nuclear
Three-stage, solid propellant
9,600 km
In Service

Peacekeeper Development

The Peacekeeper program began in 1971 as the missile experimental (MX) system as a way to increase the U.S. counterstrike capabilities against the Soviet Union, which at the time was focusing on constructing hardened shelters and missile defense systems. The Peacekeeper employed an advanced guidance system, a Multiple Independently Targetable Reentry Vehicle (MIRV) system of a dozen warheads, and a cold launch system to allow for silo reuse.1

Full-scale missile development began in 1974, and the first flight test took place in 1983.2

While there were plans for 100 of the silo-based missiles, that number was cut to 50 in 1984. The remaining missiles were intended for deployment on railcar launchers. The Peacekeeper was first deployed in 1986 at Warren Air Force Base in Wyoming. A total of 114 missiles were produced by the end of 1988. In 2002, it was declared that the Peacekeeper would be phased out of service from October 2002 to December 2005.3


The Peacekeeper had a range of 9,600 km while carrying 10 MIRV warheads. Although it could carry as many as 12 warheads, the Strategic Arms Reduction Treaty (START I) restrictions prevented the number of warheads from exceeding 10. Each warhead employed a Mk 21 reentry vehicle with a 300-475 kT yield W87 nuclear warhead. The system was equipped with an inertial navigation system that provided it warheads with an accuracy of 90 m CEP. The missile was relatively compact for its capabilities, measuring 21.8 m in length, 2.34 m in diameter, and weighing 87,750 kg at launch. It had a three-stage solid propellant design.4

Service History

The Peacekeeper missile was originally designed to destroy hardened targets using its MIRV warheads, high individual warhead yield, increased accuracy, and upgraded penetration aids. Functioning as a complement to the Minuteman series, the original 50 Peacekeeper missiles each carried 10 warheads and were intended to destroy large numbers of Soviet missile silos, decreasing Soviet second-strike options.5

Before the Peacekeeper was deployed, nearly a dozen different basing options were explored for the missile.  These options included a variety protective shelters, anti-ballistic missile defenses, or even basing on submarines or aircraft.6 Eventually, it was decided to deploy the Peacekeeper in modified Minuteman silos and railcar launch vehicles, though the railcar basing was never implemented. The missile could be launched within several minutes of a nuclear strike, had emergency airborne launch controllers which allowed the missile to be retargeted midflight, and was protected from nuclear strikes through its hardened silo and shock-protected launch canister.7

The Peacekeeper’s cold-launch system was built to eject the missile from the silo using steam pressure before engine ignition, which minimized damage to silos and allowed them to be reloaded with another missile.8

In the event of a first-strike scenario which disabled many U.S. missile silos, a small number of surviving Peacekeeper missiles were expected to be capable of dealing a tremendous amount of damage to an enemy nation. This greatly increasing the reprisal cost to a nation attacking the United States. Combined, the Peacekeeper’s reliability and survivability made it extremely effective as a counterstrike weapon.

The decision to remove the Peacekeeper missile from service was a combination of financial considerations and significant policy debate regarding the effects of MIRV capable missiles on the nuclear balance. Dr. John Foster, former Director of Defense Research and Engineering for the Pentagon, advocated for deployment of the Peacekeeper and expressed his views via letter to U.S. Senator Edward Brook saying: “Our development of MIRV is, in essence, a reaction to Soviet ABM activity…Faced with these Soviet defense measures, our deterrent capability could erode seriously without a countering move on our part…Our own MIRV systems are not efficient against silos; they are designed for use against defended urban-industrial type targets.  Other arguments for the Peacekeeper claimed that in addition to being necessary for ABM penetration, the missile enhanced the United States’ deterrence capability because it was to be a mobile, rail-based system which could stay on the move in periods of heightened tension.

Opponents of the Peacekeeper, such as Stansfield Turner, a former Director of National Intelligence and Navy admiral, considered the missile to be a destabilizing factor because they believed its capabilities could potentially threaten Soviet counterstrike capability.  Admiral Turner stated: “the MX added to the capability of our existing ICBMs – would give us the potential for a surprise attack on Soviet ICBMs, it would make the Russians nervous, their finger too, would have been on the trigger…we must commit ourselves to a doctrine of assured retaliation, relying principally on submarine-based missiles, bombers, and cruise missiles, and rejecting the MX as unsuited to our needs.”9 Essentially, opponents of the MX believed that the introduction of the Peacekeeper might create a “use it or lose it” mentality within Soviet Union, which could lead to a Soviet first strike because of the perception that Soviet missiles were vulnerable to a first strike by the United States.

The United States ultimately phased out the missile as a part of the START II arms reduction agreement with Russia, switching to a missile force made up entirely of Minuteman IIIs.10


    1. Duncan Lennox, “LGM-118 Peacekeeper” in Jane’s Strategic Weapon Systems (London: IHS Janes, 2011).
    2. Thomas Cochran, U.S. Nuclear Forces and Capabilities, (Cambridge: Ballinger Publishing Company, 1984), 125.
    3. Lennox.
    4. Ibid.
    5. Ibid.
    6. Office of Technology Assessment, “MX Missile Basing,” (Washington D.C: Office of Technology Assessment), 3-7.
    7. Cochran, 128.
    8. National Park Service, “MX Peacekeeper ICBM,” Accessed 11/18/2016, https://www.nps.gov/mimi/learn/historyculture/mx-peacekeeper-icbm.htm.
    9. Ibid, 174.
    10. Ibid, 172-173.
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Missile Defense Project, "LGM-118 Peacekeeper (MX)," Missile Threat, Center for Strategic and International Studies, February 15, 2017, last modified April 23, 2024, https://missilethreat.csis.org/missile/lgm-118-peacekeeper-mx/.