The United States and Soviet Union first started developing intercontinental ballistic missiles (ICBMs) in the 1950s, because there was no way for a target to defend against them. Missile warheads are so small and travel so fast that it is virtually impossible for a defensive weapon to hit them-a fact that remains almost as true today as it was during the 1950s. But for decades, the United States and Russia have spent tremendous amounts of money trying to develop a defence against ballistic missiles.

 
The Strategic Air Command's first intercontinental ballistic missile (ICBM) was the Convair B-65 Atlas (later redesignated SM-65). The Atlas became operational in 1959. Because of the vulnerability of the Atlas while above ground, an underground silo was developed. An elevator raised it to ground level for launching. While on alert duty, the Atlas missile was maintained in the fully raised (above ground) position since it could not be launched from its underground silo.

The first serious study of what was called an “anti-missile missile” dates to as early as 1956, when a U.S. scientific group evaluated the challenges of shooting down ballistic missile warheads. They realized that warheads were small and might not show up on radar, and responding to an attack in time would be difficult. But the biggest problem was getting a missile into the vicinity of the attacking warhead and destroying it. Because they could not get close to their targets, early anti-missile proposals all relied on nuclear warheads, which had a wide enough explosive radius that they could compensate for the inaccuracy of the missile. In the mid-1950s, the U.S. Army built an “anti-ballistic missile” (ABM) known as the Nike-Zeus with some anti-missile capabilities, but by 1959, President Dwight D. Eisenhower's Presidential Science Advisory Committee (PSAC) ruled that Nike-Zeus was too slow, too vulnerable to attack, and could not differentiate between real warheads and decoys.

By the early 1960s, U.S. Air Force scientists had evaluated the possibility of using lasers to burn missile warheads in flight but determined that the lasers could not produce enough energy to damage a warhead, which was small and fast and already designed to withstand tremendous heat during re-entry into the Earth's atmosphere. The U.S. Army began developing new radars encased in hardened structures and a faster missile named the Sprint; the new radars and missile were combined into a system named Nike-X. In 1966, the highest level military leaders recommended that this system be deployed to defend the entire United States against Soviet ICBM attack, a so-called “area defence.” But Secretary of Defence Robert McNamara resisted. Instead, McNamara pushed the development of independently targetable re-entry vehicles, or MIRVs, which allowed a single ICBM to attack multiple targets. This assured that no attack on the United States could destroy a large enough number of American ICBMs so as to prevent it from staging a devastating counterattack, and supposedly made a Soviet ICBM attack unthinkable by the Soviet military.

 
Titan I ICBM launch at Vandenberg Air Force Base, December, 1960

In late 1967, McNamara agreed to start work on a modified, scaled back version of the Nike-X system dubbed Sentinel. McNamara argued that this reduced system would focus on the Chinese missile threat rather than the Soviet threat and as such, would be less provocative. Sentinel would have resulted in basing large numbers of powerful nuclear warheads atop Spartan missiles around major American cities.

In early 1969, the new administration under President Richard Nixon re-evaluated the Sentinel system and scaled it back dramatically, renaming it Safeguard. Instead of an area defense system, Safeguard would be a “point defence” system intended to protect ICBM sites. By 1970, both the United States and Soviet Union had approximately the same number of ICBMs, and the two countries started discussing arms control, signing the Strategic Arms Limitation Treaty (SALT) in 1971. In May 1972, they both signed the Anti-Ballistic Missile (ABM) Treaty. This treaty limited both countries to two ABM sites each, one to protect the Nation's capital and the other to protect an ICBM site. It also restricted testing of systems capable of shooting down ICBMs as well as deployment of new ABM radars deep inside a Nation's territory.

Throughout the 1950s and 1960s, while the United States was developing its ABM systems, the Soviet Union was also conducting a massive ABM development program. The Soviet military tested huge radars and powerful missile interceptors at a big facility known as Sary Shagan. But despite all of their efforts, the Soviets were unable to develop an effective missile defence system. They started a system for defending Moscow in the 1960s and eventually completed it after a number of setbacks.

 
Boeing LGM-30A Minuteman I. The Minuteman I, formally known as the SM-80, was a second generation intercontinental ballistic missile (ICBM) using solid propellants rather than liquid fuels.

In early 1975, the United States deployed its ABM system at Grand Forks, North Dakota, to defend ICBM silos. But Congress closed the facility within a year because there was no way that its 100 missile interceptors could defend against thousands of incoming Soviet warheads. The Soviet Union maintained its limited ABM system around Moscow and updated it starting in the 1980s. But the system never worked properly. Critics within the Soviet military noted that the defensive system itself would detonate dozens of nuclear weapons directly over the city. Proponents of the system rationalized that the system was intended to defend against a much more limited missile attack from China, but outdated technology made even this reduced task seem nearly impossible.

On March 23, 1983, President Ronald Reagan gave a speech announcing a major shift in American defence policy. Reagan declared that the United States would seek to develop a missile shield to defend the entire country against Soviet ICBM attack. This shield would rely upon many highly advanced and unproven technologies, such as lasers and particle beams, and much of it would presumably be deployed in space. The lasers would attack Soviet ICBMs while they were still relatively slow and lifting off the ground, when they were most vulnerable (a period known as the “boost phase”). The Soviets had also started developing a large radar deep inside their territory near Krasnoyarsk that directly violated the ABM Treaty. When the Americans discovered this in July 1983, relations deteriorated.

Critics immediately labelled Reagan's defence plan “Star Wars” and ridiculed it for being totally unrealistic. Reagan soon established the Strategic Defence Initiative, or SDI, to develop the advanced technologies necessary for effective missile defence. The SDI budget grew to nearly three billion dollars a year, but even so, many of its most advanced technologies proved beyond reach. In the late 1980s, SDI plans were scaled back. Instead of lasers, one proposed solution was to use thousands of small orbiting interceptors nicknamed “Brilliant Pebbles.” These would be combined with many small orbiting sensors nicknamed “Brilliant Eyes.” Both proposals ultimately led to a new approach to spacecraft design: an effort to develop smaller, cheaper spacecraft. Despite the expenditure, SDI did not make the kind of progress necessary to achieve Reagan's grand vision, but it did greatly concern Soviet military and government leaders, who often had more faith in American technology than the Americans did themselves.

President George H.W. Bush scaled back the SDI program after the end of the Cold War but still sought to develop some form of missile defence. The Iraqi use of Scud missiles during the 1991 Persian Gulf War demonstrated that American troops abroad were also at risk from missile attack. This threat prompted greater focus on so-called “theater” ballistic missile defences to shoot down shorter-range missiles like the Scud, which are slower and easier to hit than ICBMs.

After Bill Clinton defeated Bush in the 1992 presidential election, he scaled back the effort even further and focused it on developing several ground and sea-based interceptors for defending against a small attack from nations such as North Korea, Iraq, and Iran. These interceptors would directly hit their targets at high speed and did not need a warhead. The U.S. Air Force also began work on an Airborne Laser (ABL) that would be mounted on a converted Boeing 747 airplane and used to shoot down missiles like the Scud. The Strategic Defence Initiative Organization was renamed the Ballistic Missile Defence Organization (BMDO).

For the next eight years, Clinton continued missile defence research at a slower pace. But in 1998, North Korea surprised the world by launching a three-stage ballistic missile over Japan. This prompted renewed debate within the United States about the threat from ballistic missiles, and presidential candidate George W. Bush made missile defence a key part of his campaign. When Bush was sworn in as president in January 2001, he quickly moved to increase missile defence development, pushing deployment of a small system based in Alaska that could intercept a small number of ICBMs launched at the continental United States. In December 2001, Bush announced his intention to withdraw the United States from the ABM Treaty. Such a move was necessary if the United States was going to test more advanced systems that would otherwise violate the treaty.