Nike-X

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. This was fine in the late 1950s when the Soviets had only a few dozen missiles, but by the 1960s it was predicted they would have hundreds and could 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.