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.

Initial plans called for building and deploying 100 MX ICBMs, but budgetary concerns limited the final procurement; only 50 entered service.

The first flight test took place in 1983, which included the successful launch of six inert re-entry vehicles, each hitting pre-planned targets.

At this time, the United States and the Soviet Union were negotiating the START II treaty, under which ICBMs were allowed to carry only a single warhead each.

Because the Minuteman could carry a single warhead for far less money, the United States agreed to remove the Peacekeeper from their nuclear force in this treaty.

Limited accuracy with a circular error probable (CEP) of about 0.6 to 0.8 nautical miles (1.1 to 1.5 km)[11]: 205  and a small warhead of less than 1 megaton meant the system was unable to attack hardened targets like missile silos.

[12]: 123 As the US Navy was quick to point out, their Polaris fleet's stealth and mobility would maintain a countervalue force under any possible scenario.

The Air Force could not let this stand, and following the advice of RAND Corporation, in 1962 they decided the solution was to make the Minuteman capable of counterforce missions as well.

One was to introduce the NS-17 inertial navigation system, which improved the CEP to 0.34 nautical miles (0.63 km) and allowed the missiles to attack Soviet silos directly.

In 1964, the Air Force contracted them to consider a wide variety of survivable ICBM approaches, under the name "Golden Arrow".

The proposal called for an enormous (for the day) turboprop-powered aircraft with two-day endurance carrying up to eight missiles that would be dropped out the back, parachuted to the vertical, and then launched.

[12]: 131  As part of the same study, Aerospace also considered a missile and wheeled launcher combination that was small enough that they could be carried in existing C-141 Starlifter aircraft.

[12]: 133 Finally, they also considered conventional missiles in "super hard" silos, buried under the southern side of mountains.

[12]: 136 Any of the "Golden Arrow" concepts would be extremely expensive, and in the era of Robert McNamara's US Department of Defense, cost was as important as any other consideration.

If we attempted to develop and procure a dozen or more distinct different nuclear delivery systems… we doubtless would end up squandering our resources and not doing a good job on any of them.

Although the system did not include the ability to be rapidly retargeted, this capability was under development and started deployment in 1972, before the planned 1975 introduction date of WS-120A.

[12]: 140 Since the late 1950s, engineers at the Charles Stark Draper Laboratory had been working on a new type of inertial platform that replaced the mechanical gimbals with a sphere floating in a thin layer of fluorocarbon fluid.

The name referred to the concept that the system would be so accurate and free from the effects of mechanical shocks and jarring that it would not require any other form of "fixing" in flight.

In some ways this helped the Air Force, as it meant they could concentrate on the counterforce scenarios, knowing that a countervalue attack would always be available from the Navy.

However, improvements in SLBM accuracy might allow them to handle counterforce as well, and render the entire land-based ICBM fleet superfluous.

In 1971, the Air Force started a requirements development process combining the ICBM-X and SABRE concepts into a single platform, "Missile, Experimental", or MX.

In November, the Secretary of Defense pushed the initial operational date back from 1983 to 1985 and opened a study on the possibility of developing a single missile for both ICBM and SLBM use.

The deployment would occur in a system of multiple protective shelters linked by underground or aboveground roads, the so-called "Racetrack" proposal.

On 10 August, the Secretary of Defense ordered 100 Peacekeepers be deployed at Warren AFB in Wyoming and began development of what became the MGM-134 Midgetman.

The development of the Trident II, which was described as "effective against most of the hardened military targets, including missile silos and launch control centers",[21] reopened the debate about MX.

The supposed counterforce gap, then being widely talked about on television, also resulted in the schedule for silo deployment being moved up, dropping the production time from 44 months to 29.

The railcars were intended, in case of increased threat of nuclear war, to be deployed onto the nation's rail network to avoid being destroyed by a first strike counterforce attack by the Soviet Union.

However the plan was cancelled as part of defense cutbacks following the end of the Cold War, and the Peacekeeper missiles were installed in silo launchers as LGM-118s instead.

A Congressional report stated that "Northrop was behind schedule before it even started" and noted that the Air Force knew as early as 1985 that there were "serious system deficiencies as well as a lack of effective progress".

[2] The Peacekeeper rockets were converted to the satellite launcher role by Orbital Sciences, as the Minotaur IV (OSP-2), while the warheads were redeployed on the existing Minuteman III missiles.