Prim–Read theory, or Prim–Read defense, was an important development in game theory that led to radical changes in the United States' views on the value of anti-ballistic missile (ABM) systems. The theory assigns a certain cost to deploying defensive missiles and suggests a way to maximize their value in terms of the amount of damage they could reduce. By comparing the cost of various deployments, one can determine the relative amount of money needed to provide a defense against a certain number of ICBMs. The theory was first introduced in the late 1950s and might have been lost to history had it not been picked up during the debate on the Nike-X ABM. Nike-X called for the deployment of a heavy defensive system around major US cities with the intent of seriously blunting the effect of any Soviet strike. A number of operations researchers, notably US Air Force General Glenn Kent, used Prim–Read to conclusively demonstrate that the cost of reducing damage back to a given level was always more than the cost of causing additional damage by building more ICBMs. The outcome of these studies suggested that any US deployment of an ABM system would result in the USSR building a small number of additional missiles to defeat it. Assuming the Soviets would come to the same conclusion, Robert McNamara became highly critical of any large-scale ABM system, and began efforts that would ultimately lead to the ABM treaty in 1972. The underlying concept became known as the cost-exchange ratio.
History
Nike Zeus
The US Army began studying the anti-ballistic missile in a serious fashion in 1955. Working with Bell Labs, who had delivered the successful Nike and Nike B surface-to-air missiles (SAM), they began by considering what was essentially a direct update of the Nike concepts to the ABM mission. Bell returned a report suggesting that minor upgrades to the Hercules missile, along with much more powerful radars and computers, would do the trick. This was initially known as Nike II, but renamed Nike Zeus in 1956. Early in the Zeus effort the US Air Force attempted to derail the project by pointing out that if Zeus cost the same as an ICBM, and the Soviets were building them as quickly as Nikita Khrushchev claimed, then the Soviets would simply build a few more to "soak up" any Zeus' the Army deployed. But in fact, it seemed the ICBMs were actually cheaper than Zeus, perhaps significantly, which meant the US would lose the resulting arms race. This basic concept became known as the cost exchange ratio. President Eisenhower's Secretary of Defense Neil McElroy identified the Air Force complaints as an example of sour grapes, having lost funding for their own ABM efforts, Project Wizard, in favor of Zeus. But the math appeared to be correct, so he asked for a second opinion from the President's Science Advisory Committee (PSAC). They largely agreed with the Air Force's take, and then added several additional concerns of their own. By the late 1950s, several new problems became evident. One was that the newly discovered nuclear blackout effect would allow an enemy to blanket an area hundreds of miles wide with a radar-opaque layer for the cost of one warhead. This would render Zeus blind to anything above the layer; following warheads would not become visible under the cloud until they were too close to impact for the Zeus to fly up to them. Another issue was the addition of decoys to the ICBMs, which presented radar targets that looked the same as the warhead. These cleared away due to drag as they reentered the atmosphere, but once again, this occurred at too low an altitude to attack.
Nike-X
At the suggestion of ARPA, the Army responded by redesigning the system as Nike-X. Nike-X used a short-range but extremely high-speed interceptor known as Sprint that was optimized for interceptions under 60 kilometres (200,000 ft) and combined that with an extremely high-speed radar and computer system. The plan was to wait until the warhead cleared any blackout and the decoys were slowing, allowing the radar to pick out the warhead and attack it with the Sprint. The entire engagement would last only a few seconds. The Army produced a study that considered a real deployment scenario and then estimated the number of lives it would save. They started by assuming that the Soviets would want to launch two warheads at every target, to ensure at least one would go off. In order to confuse the defense, they would add nine credible decoys to each ICBM. This would present each base with 20 radar targets in total. For the same redundancy reasons, the Army would launch two Sprint missiles at each target, so a total of 40 Sprints would be needed to protect every target. Given the relative costs of the Sprint and an ICBM, the Army demonstrated that the Sprint system would save a considerable number of civilian lives for less than the cost of an ICBM.
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