http://en.wikipedia.org/wiki/LGM-118_Peacekeeper
LGM-118 Peacekeeper
The LGM-118A Peacekeeper, initially known as the "MX missile" (for Missile-eXperimental), was a land-based ICBM deployed by the United States starting in 1986. A total of 50 missiles were deployed.
During the late 1970s, the Soviet Union fielded a large number of heavy, increasingly accurate, MIRVed ICBMs (like the SS-18, defined by throw weight) which seriously threatened the survival of Minuteman III missiles in their silos. If the Soviet Union could have knocked out the majority of these missiles and the strategic bombers on ground, it would have left the United States with no counterforce capability for the second strike. The Poseidon and Trident I SLBMs were not accurate enough for counterforce strikes, and did not carry high enough yield. If the Soviet first strike would have avoided hitting civil targets, the US might have been forced not to retaliate against Soviet cities, because of similar Soviet countercity capabilities. So the United States needed weapons which could survive a Soviet first-strike and neutralize the remaining Soviet strategic arsenal, in order to avoid nuclear blackmail.
The development of the Peacekeeper began with the intent of its being a counterforce, hard-target weapon. It was to be aimed at hardened enemy missile silos with first-strike capability. This required high accuracy, survivability, range and a flexibility that was not available in the Minuteman III.
Accuracy was an especially important issue, since the effects of the blast and heat from a thermonuclear detonation decrease very rapidly with distance from the impact point. The targets were hardened Soviet ICBM-silos, especially those housing the heavy SS-18, which were targeted at Minuteman missile silos.
The survivability problem was to be solved by a "rail garrison" system whereby 25 trains, each with two missiles, would use the national railroad system to conceal themselves.
