MIRV stands for Multiple Independently-targeted Re-entry Vehicle. This is a technology that allows multiple warheads and/or dummies and/or counter-countermeasures to be placed on the warhead bus of an ICBM. IIRC, the first U.S. ICBM to be MIRVed was the Minuteman III. MIRV does indeed make missiles more economical, but has implications beyond that.

Originally, ICBMs were horribly inaccurate, with CEP ratings measured in miles. Note, this doesn't matter much when your target is a city (countervalue targeting); it also is no mean feat to place a purely ballistic object within a couple of miles of its target on the other side of a 7k-plus-mile diameter rotating sphere. However, when shooting at other missile silos (known as counterforce), although the Minuteman-III had a CEP of probably around 1/2 mile to 1/4 mile, when shooting at a silo hardened to resist 2000 psi of blast overpressure, this may not be close enough. The MIRV allows targeters to allocate multiple warheads to the same target in pursuit of greater Pk (standing for 'Probability of Kill')

In addition, warheads from each missile could cover a number of targets, thus in the event that a missile failed to function properly, 'adequate' target coverage could still be maintained even if you didn't know which one failed. With unitary warheads, redundant coverage always means building more missiles, and silos, and crews to man them, and warheads, and thus radioactives, etc. etc.

MIRV became especially popular after the SALT I treaty was signed and ratified. This is because that treaty limits the number of nuclear-capable launchers a nation can have, not the number of warheads. So a missile with one warhead counts the same as a missile with ten warheads. Although subsequent agreements (START,INF) have addressed the warhead count, at the time, MIRV was an excellent way to conserve launchers by lowering the number of ICBMs in the nuclear forces.

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