Nike-X was a response to the failure of the earlier Nike Zeus system. Zeus had been designed to face a few dozen Soviet ICBMs in the 1950s, and its design would mean it was largely useless by mid-1960s when it would be facing hundreds. It was calculated that a salvo of only four ICBMs would have a 90% chance of hitting the Zeus base, as their radars could only track a few warheads at the same time. Worse, the attacker could 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.