Wikipedia
Nike-X was an anti-ballistic missile (ABM) system designed by the US Army to protect major cities in the United States from attacks by the Soviet Union's intercontinental ballistic missile (ICBM) fleet during the Cold War. The X in the name referred to its experimental basis, and was supposed to be replaced by a more appropriate name when the system was put into production. This never came to pass; the Nike-X program was canceled and replaced by a much lighter defense system known as Sentinel.
The system was developed in response to the failure of the earlier Nike Zeus system. It was calculated that a salvo of only four ICBMs would have a 90% chance of hitting the Zeus base, whose radars could only track a few objects at the same time. Zeus was useful in the late 1950s when the Soviets had only a few dozen missiles, but with their fleet predicted to have of hundreds by the early 1960s they could easily afford to overwhelm Zeus. The attacker could also use radar reflectors or high-altitude nuclear explosions to obscure the warheads until they were too close to attack, making a single warhead attack highly likely to succeed.
The key concept that led to Nike-X was that the rapidly thickening atmosphere below about altitude disrupted the decoys and explosions. Nike-X intended to wait until the enemy warheads descended below this altitude and then attack them using a very fast missile known as Sprint. The entire engagement lasted only a few seconds, and could take place as low as . To provide the needed speed and accuracy, as well as deal with multi-warhead attacks, Nike-X used a new radar system and building-filling computers that could track hundreds of objects at once and controlled salvos of many Sprints. Dozens and dozens of ICBMs would need to arrive at the same time in order to overwhelm the system.
A complete deployment would be extremely expensive to build, on the order of the total yearly budget of the Department of Defense. Robert McNamara felt the cost could not be justified and worried it would lead to a further nuclear arms race. He directed the teams to consider deployments where a limited number of interceptors might still be militarily useful. Among these, the I-67 concept suggested building a lightweight defense against very limited attacks. When the People's Republic of China exploded their first H-bomb in 1967, I-67 was promoted as a defense against a Chinese attack, and this system became Sentinel in October. Nike-X development, in its original form, ended.