The US Army had considered the issue of shooting down theater ballistic missiles of the V-2 missile type as early as the mid-1940s. Early studies suggested their short flight times, on the order of 5 minutes, would make it difficult to detect, track and shoot at these weapons. However, in spite of their much higher performance, intercontinental ballistic missiles' longer flight times and higher trajectories made them, theoretically, much easier to attack.
In 1955 the Army gave Bell Labs, who had developed the earlier Nike missiles, a contract to study the ABM issue. The returned a report saying the concept was within the state of the art, and could be built using modest upgrades to the latest Army surface-to-air missile, the Nike Hercules. The main technological issues would be the need for extremely powerful radars that could detect the incoming ICBM warheads long enough in advance to fire on them, and computers with enough speed to develop tracks for the targets in engagements that lasted seconds.
Bell began development of what became Nike Zeus in 1956, working out of the Nike development center at Redstone Arsenal. The program went fairly smoothly, and the first tests were carried out in the summer of 1959. By 1962 a complete Zeus base had been built on Kwajalein Island and proved very successful over the following year, successfully intercepting test warheads and even low-flying satellites.
During the period Zeus was being developed, a number of problems arose that appeared to make it trivially easy to defeat. The simplest was that its 1950s-era mechanical radars could track a limited number of targets, and it could be easily overwhelmed by numbers; a report by the Gaither Committee suggested a salvo of four warheads would have a 90% chance of destroying a Zeus base. This was of little concern during early development when ICBMs were enormously expensive, but as their cost fell and the Soviets claimed to be turning them out "like sausages", this became a serious problem.
But other issues also became obvious in the late 1950s. One issue was that nuclear explosions in space had been tested in 1958 and found that they blanketed a huge area with radiation that blocked radar signals above about 60 kilometres (37 mi) altitude. By exploding a single warhead above the Zeus sites, the Soviets could block observation of following warheads until they were too close to attack. Another simple measure would be to pack radar reflectors in with the warhead, presenting many false targets on the radar screens that cluttered the displays.
As the problems piled up, the Secretary of Defense Neil H. McElroy asked ARPA to study the anti-missile concept. ARPA noted that both the radar decoys and high-altitude explosions both stopped working in the thickening lower atmosphere. If one simply waited until the warheads descended below about 60 km, they could be easily picked out on radar again. However, as the warheads would be moving at about 5 miles (8.0 km) per second at this point, they were only seconds from their targets. An extremely high-speed missile would be needed to attack them during this period.
The result of the ARPA study came at the height of the debate over the Zeus system in the early 1960s. The new Secretary of Defense, Robert McNamara, convinced President Kennedy that Zeus was simply not worth deploying. He suggested using the funds allocated to its deployment to develop the ARPA system, which became known as Nike-X, an ad hoc name given by Jack Ruina when he was reporting on the concept.
Nike-X required great improvements in radars, computers, and especially the missile. Zeus had an attack profile lasting about a minute, Nike-X's interceptions would last about five seconds.