Missile defense
In the 1950s, the first United States anti-ballistic missile system was the Nike Hercules, which had the ability to intercept incoming short-range ballistic missiles, but not intermediate-range ballistic missiles (IRBMs) or ICBMs. This was followed by the Nike Zeus, which was capable of intercepting ICBMs by using a nuclear warhead, upgraded radar systems, faster computers, and control systems that were more effective in the upper atmosphere. However it was feared the missile's electronics may be vulnerable to x-rays from a nuclear detonation in space. A program was started to devise methods of hardening weapons from radiation damage. By the early 1960s the Nike Zeus was the first anti-ballistic missile to achieve hit-to-kill (physically colliding with the incoming warhead).
In 1963, Secretary of Defense Robert McNamara diverted funds from the Zeus missile program, and instead directed that funding to the development of the Nike-X system, which utilized the high-speed, short-range Sprint missile. These missiles were meant to intercept incoming warheads after they had descended from space and were only seconds from their targets. To accomplish this, Nike-X required advances in missile design to make the Sprint missile quick enough to intercept incoming warheads in time. The system also included advanced active electronically scanned array radar systems and a powerful computer complex.
During the development of Nike-X, controversy over the effectiveness of anti-ballistic missile systems became more prominent. Critiques of the Nike-X included an estimate that the anti-ballistic missile system could be defeated by Soviets manufacturing more ICBMs, and the cost of those additional ICBMs needed to defeat Nike-X would also cost less than what the United States would spend on implementing Nike-X. Additionally, McNamara reported that a ballistic missile system would save American lives at the cost of approximately $700 per life, compared to a shelter system that could save lives at a lower cost of approximately $40 per life. As a result of these estimations, McNamara opposed implementation of Nike-X due to the high costs associated with construction and perceived poor cost-effectiveness of the system, and instead expressed support for pursuing arms limitations agreements with the Soviets. After the Chinese government detonated their first hydrogen bomb during Test No. 6. in 1967, McNamara modified the Nike-X program into a program called Sentinel. This program's goal was to protect major US cities from a limited ICBM attack, especially on one from China.
This would be done by building fifteen sites across the continental US, and one site in each of Alaska and Hawaii. This in turn reduced tensions with the Soviet Union, which retained the offensive capability to overwhelm any US defense. McNamara favored this approach as deploying the Sentinel program was less costly than a fully implemented Nike-X program, and would reduce congressional pressures to implement an ABM system. In the months following the announcements regarding the Sentinel program, Secretary of Defense Robert McNamara stated: "Let me emphasize—and I cannot do so too strongly—that our decision to go ahead with a limited ABM deployment in no way indicates that we feel an agreement with the Soviet Union on the limitation of strategic nuclear offensive and defensive forces is in any way less urgent or desirable.

With the conclusion of the Cuban Missile Crisis and the withdrawal of Soviet missiles from their strategic positions in Cuba, the USSR started to think about a missile defense systems. A year after the crisis in 1963 the Soviets created the SA-5. Unlike its predecessors like the SA-1 or Griffon systems, this system was able to fly much higher and further  and was fast enough to intercept some missiles however its main purpose was to intercept the new XB-70 supersonic aircraft the U.S was planning to make. However, since these types of aircraft never went into production in the U.S, the project was abandoned, and the Soviets reverted to the slower, low altitude SA-2 and SA-3 systems. In 1964 the Soviets publicly unveiled their newest interceptor missile named the "Galosh" which was nuclear armed and was meant for high altitude, long range interception. The Soviet Union began installing the A-35 anti-ballistic missile system around Moscow in 1965 using these "Galosh" missiles and would become operational by 1971. It consisted of four complex around Moscow each with 16 launchers and two missile tracking radars. Another notable feature of the A-35 was that it was the first monopulse radar. Developed by OKB 30, the Russian Special Design Bureau, the effort design to create a monopulse radar started in 1954. This was used to conduct the first successful intercept in 1961. There were known flaws with the design such as an inability to defend against MIRV and decoy style weapons.The reason for this was because the detonation of a nuclear interceptor missile like the "Galosh" creates a cloud of plasma that temporarily impairs radar readings around the area of the explosion limiting these kinds of systems to a one-shot capacity. This means that with MIRV style attacks the interceptor would be able to take out one or two but the rest would slip though. Another issue with the 1965 model was that it consisted of 11 large radar stations at six locations on the borders of Russia. These bases where visible to the US and could be taken out easily leaving the defense system useless in a concentrated and coordinated attack. Finally, the missiles that could be held on each base was limited by the ABM treaty to only 100 launchers maximum, meaning that in a massive attack they would be depleted quickly. During installation, a Ministry of Defense commission concluded that the system should not be fully implemented, reducing the capabilities of the completed system. That system was later upgraded to the A-135 anti-ballistic missile system and is still operational. This upgrade period started in 1975 and was headed by Dr. A.G. Basistov. When it was completed in 1990, the new A-135 system had a central control multifunctional radar called the "Don" and 100 interceptor missiles. Another improvement was the layering of interceptor missiles where high acceleration missiles are being added for low flying targets and the "Galosh" style missiles where improved further for high altitude targets. All of these missiles where moved underground into silos to make them less vulnerable, which was a flaw of the previous system.
The SALT I talks began in 1969, and led to the Anti-Ballistic Missile Treaty in 1972, which ultimately limited the US and USSR to one defensive missile site each, with no more than 100 missiles per site. This included both ABM interceptor missiles as well as launchers. Originally, the agreement made by the Nixon administration and the Soviet Union stated that both of the two nations were each allowed to have two ABM defensive systems present in their own countries. The goal was to effectively have one ABM defense system located near each nation's capital city as well as another ABM defense system placed near the nation's most important or strategical ICBM field. This treaty allowed for an effective form of deterrence for both sides as if either side were to make an offensive move, the other side would be capable of countering that move. However, a few years later in 1974 both sides reworked the treaty to include only one ABM defensive system present around an ICBM launch area or the nation's capital city.  This occurred once both sides determined the other side was not going to construct a second ABM defensive system. Along with limiting the amount of ballistic missile defense systems each nation could have, the treaty also stated if either country desired to have a radar for incoming missile detection, the radar system must be located on the outskirts of the territory and must be aligned in the opposite direction of one's own country. This treaty would end up being the precedent set for future missile defense programs, as any systems that were not stationary and land-based were a violation of the treaty.
As a result of the treaty and of technical limitations, along with public opposition to nearby nuclear-armed defensive missiles, the US Sentinel program was re-designated the Safeguard Program, with the new goal of defending US ICBM sites, not cities. The US Safeguard system was planned to be implemented in various sites across the US, including at Whiteman AFB in Missouri, Malmstrom AFB in Montana, and Grand Forks AFB in North Dakota. The Anti-Ballistic Missile Treaty of 1972 placed a limit of two ABM systems within the U.S, causing the work site in Missouri to be abandoned, and the partially-completed Montana site was abandoned in 1974 after an additional agreement between the US and USSR that limited each country to one ABM system. As a result, the only Safeguard system that was deployed was to defend the LGM-30 Minuteman ICBMs near Grand Forks, North Dakota. However, it was deactivated in 1976 after being operational for less than four months due to a changing political climate plus concern over limited effectiveness, low strategic value, and high operational cost.

In the early 1980s, technology had matured to consider space based missile defense options. Precision hit-to-kill systems more reliable than the early Nike Zeus were thought possible. With these improvements, the Reagan administration promoted the Strategic Defense Initiative, an ambitious plan to provide a comprehensive defense against an all-out ICBM attack. In pursuit of that goal, the Strategic Defense Initiative investigated a variety of potential missile-defense systems, which included systems utilizing ground-based missile systems and space-based missile systems, as well as systems utilizing lasers or particle beam weapons.