The Spartan was the latest and, as it turned out, final development in a long series of missile designs from the team of Bell Laboratories and Douglas Aircraft Company that started in the 1940s with the Nike.

They returned an initial study on Nike II in January 1956, Concluding that the fundamental concept was feasible, utilizing a slightly enhanced iteration of the Hercules missile, but requiring dramatically upgraded radars and computers to handle interceptions that took place at thousands of miles an hour.

Eventually fourteen "all up" tests were carried out over the next two years, with ten of them bringing the missile within the lethal radius of its warhead, sometimes within a few hundred meters.

In spite of Zeus' successful testing program and interceptions, it was becoming increasingly clear that the fully integrated system would not be effective in an actual operational scenario.

This was due primarily to two problems; decoys shielding the warhead from detection until it was too late for interception, and the rapid increase in the number of deployed ICBMs which threatened to overwhelm the system.

At the same time, both the US and USSR were in the midst of introducing their first truly mass-produced ICBMs, and their numbers were clearly going to grow dramatically during the early 1960s.

Zeus, like Hercules and Ajax before it, used mechanically directed radar dishes that could track only one target and one interceptor at a time.

As the ICBM fleet numbered hundreds even before Zeus could become operational, it would be simple to overcome the defense by directing sufficient warheads over it to overwhelm its ability to guide interceptions rapidly enough.

Likewise, the solution to dealing with massive numbers of warheads was to use faster computers and automated radars, allowing many interceptors to be in flight simultaneously.

After considerable debate, the decision was made to cancel the existing Zeus deployment and move ahead with the X plan.