The interception technology used has varied over time. In the 1960s, missile defense against ICBMs emphasized nuclear warheads. In recent decades non-nuclear kinetic warheads have been used. Directed-energy weapons such as lasers have been investigated and deployed on a limited basis.
The US, Russia, France, Israel and India have all developed such air defence systems. In the United States, missile defense was originally the responsibility of the Army, but in recent years the Navy and Air Force have developed their own systems.
Some missiles such as THAAD can intercept both inside and outside the earth's atmosphere, giving two intercept opportunities.
In the 1950s and 1960s, the term meant defense against strategic (usually nuclear-armed) missiles. The technology mostly centered around detecting offensive launch events and tracking in-bound ballistic missiles, but with limited ability to actually defend against the missile. The Soviet Union achieved the first nonnuclear intercept of a ballistic missile warhead by a missile at the Sary Shagan antiballistic missile defense test range on 4 March 1961.
In the late 1960s, technology had matured to the point to allow limited rudimentary missile defense by using anti-ballistic missiles. At first, these anti-ballistic missiles were armed with their own nuclear weapons, since precision technology (to "hit-a-bullet-with-a-bullet") did not exist. Lasers were considered for shooting down the warheads, but various problems including atmospheric interference and warhead hardness impeded this effort.
Throughout the 1950s and 1960s, the United States Project Nike air defense program focused initially on bombers, then ballistic missiles. In the 1950s, the first United States anti-ballistic missile system was the Nike Hercules, which had a limited ability to intercept incoming ballistic missiles, although not ICBMs. This was followed by Nike Zeus, which using a nuclear warhead could intercept ICBMs. However Nike Zeus had other limitations which prevented it being deployed. In any case, by the early 1960s the Nike Zeus was the first anti-ballistic missile to achieve hit-to-kill (physically colliding with the incoming warhead).
The Zeus missile was enhanced, and the shorter range Sprint missile was added to the Nike defense system, then called Nike-X. The system included large powerful radars and a computer complex.
Eventually, the Nike-X program was realigned and renamed Sentinel. This program's goal was to protect major U.S. cities from a limited ICBM attack, especially focusing on China. This in turn reduced tensions with the Soviet Union, which retained the offensive capability to overwhelm any U.S. defense.
The Soviet Union deployed the A-35 anti-ballistic missile system around Moscow in 1966, which also defended nearby ICBM sites. That system has been upgraded several times and is still operational. The United States announced an ABM program to protect twelve ICBM sites in 1967.
In 1967, then-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."
The SALT I talks began in 1969, and led to the Anti-Ballistic Missile Treaty in 1972, which ultimately limited the U.S. and U.S.S.R. to one defensive missile site each, with no more than 100 missiles per site.
As a result of the treaty and of technical limitations, along with public opposition to nearby nuclear-armed defensive missiles, the U.S. Sentinel program was redesignated the Safeguard Program, with the new goal of defending U.S. ICBM sites, not cities. The U.S. Safeguard system was deployed to defend the LGM-30 Minuteman ICBMs near Grand Forks, North Dakota. 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 late 1990s, and early 2000s, the issue of defense against Cruise Missiles became more prominent with the new Bush Administration. In 2002, President George W. Bush withdrew the US from the Anti-Ballistic Missile Treaty, allowing further development and testing of ABMs under the Missile Defense Agency, and allowing for deployment of interceptor vehicles beyond the single site allowed under the treaty.
There are still technological hurdles to an effective defense against ballistic missile attack. The United States National Ballistic Missile Defense System has come under scrutiny about its technological feasibility. Intercepting midcourse (rather than launch or reentry stage) ballistic missiles traveling at several miles per second with a "kinetic kill vehicle" has been characterized as trying to hit a bullet with a bullet. Despite this difficulty, there have been several successful test intercepts and the system was made operational in 2006, while tests and system upgrades continue. Moreover, the warheads or payloads of ballistic missiles can be concealed by a number of different types of decoys. Sensors that track and target warheads aboard the kinetic kill vehicle may have trouble distinguishing the "real" warhead from the decoys, but several tests that have included decoys were successful. Nira Schwartz's and Theodore Postol's criticisms about the technical feasibility of these sensors have led to a continuing investigation of research misconduct and fraud at the Massachusetts Institute of Technology
As of February 2007, the U.S. missile defense system consists of 13 ground-based interceptors at Ft Greely in Alaska, plus two interceptors at Vandenberg AFB, California. The U.S. plans to have 21 interceptor missiles by the end of 2007 The system was initially called National Missile Defense (NMD), but in 2003 the ground-based component was renamed Ground-Based Midcourse Defense (GMD).
Defending against cruise missiles is similar to defending against hostile, low-flying manned aircraft. As with aircraft defense, countermeasures such as chaff, flares, and low altitude can complicate targeting and missile interception. High-flying radar aircraft such as AWACS can often identify low flying threats by using doppler radar. Another possible method is using specialized satellites to track these targets. By coupling a target's kinetic inputs with infrared and radar signatures it may be possible to overcome the countermeasures.
In March 2008, the U.S. Congress convened hearings to re-examine the status of missile defense in U.S. military strategy.