Editing Anti-ballistic missile (section)

===American Nike-X and Sentinel===
[[Nike Zeus]] failed to be a credible defense in an era of rapidly increasing ICBM counts due to its ability to attack only one target at a time. Additionally, significant concerns about its ability to successfully intercept warheads in the presence of high-altitude nuclear explosions, including its own, lead to the conclusion that the system would simply be too costly for the very low amount of protection it could provide.

By the time it was cancelled in 1963, potential upgrades had been explored for some time. Among these were radars capable of scanning much greater volumes of space and able to track many warheads and launch several missiles at once. These, however, did not address the problems identified with radar blackouts caused by high-altitude explosions. To address this need, a new missile with extreme performance was designed to attack incoming warheads at much lower altitudes, as low as 20&nbsp;km. The new project encompassing all of these upgrades was launched as [[Nike-X]].

The main missile was [[LIM-49 Spartan]]—a Nike Zeus upgraded for longer range and a much larger 5 megaton warhead intended to destroy enemy's warheads with a burst of x-rays outside the atmosphere. A second shorter-range missile called [[Sprint (missile)|Sprint]] with very high acceleration was added to handle warheads that evaded longer-ranged Spartan. Sprint was a very fast missile (some sources{{who|date=March 2015}} claimed it accelerated to 8,000&nbsp;mph (13&nbsp;000&nbsp;km/h) within 4 seconds of flight—an average acceleration of ''90 [[g-force|g]]'') and had a smaller W66 [[Neutron bomb|enhanced radiation]] warhead in the 1–3 kiloton range for in-atmosphere interceptions.

The experimental success of Nike X persuaded the [[Lyndon B. Johnson]] administration to propose a thin ABM defense, that could provide almost complete coverage of the United States. In a September 1967 speech, Defense Secretary [[Robert McNamara]] referred to it as "[[Sentinel program|Sentinel]]". McNamara, a private ABM opponent because of cost and feasibility (see [[cost-exchange ratio]]), claimed that Sentinel would be directed not against the Soviet Union's missiles (since the [[Soviet Union|USSR]] had more than enough missiles to overwhelm any American defense), but rather against the potential nuclear threat of the People's Republic of China.

In the meantime, a public debate over the merit of ABMs began. Difficulties that had already made an ABM system questionable for defending against an all-out attack. One problem was the [[Fractional Orbital Bombardment System]] (FOBS) that would give little warning to the defense. Another problem was high altitude EMP (whether from offensive or defensive nuclear warheads) which could degrade defensive radar systems.

When this proved infeasible for economic reasons, a much smaller deployment using the same systems was proposed, namely Safeguard (described later).