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The American armed forces began experimenting with anti-missile missiles shortly after World War II, as the extent of German research into rocketry became clear. But defenses against Soviet long-range bombers took priority until the later 1950s, when the Soviets began to test their missiles (most notably via the Sputnik launch in October 1957). The first experimental ABM system was Nike Zeus, a modification of existing air defense systems. Nike Zeus proved unworkable, and so work proceeded with Nike X.
Nike X featured the Spartan and Sprint interceptors, and enjoyed some successes. Like its predecessor, Nike X used missiles tipped with nuclear warheads to destroy its targets. Sprint was a very fast missile (some sources claimed it accelerated to Mach 10 within 5 seconds of flight--an acceleration of 100 g!) and had a smaller warhead in the low kiloton range. It was designed to protect the Nike X emplacement against enemy missiles which avoided the massive (perhaps as much as 5 megaton) warhead of the Spartan interceptor.
The experimental success of Nike X persuaded the Lyndon Johnson administration to propose a thin ABM defense in a September 1967 speech by Defense Secretary Robert McNamara. McNamara, a private opponent of ABM because of cost and feasibility, claimed that the ABM system would be directed not against the Soviet Union's missiles (since the U.S.S.R. had more than enough missiles to overwhelm any American defense), but rather against the potential nuclear threat of the People's Republic of China. Cynics thought that the system was really an "anti-Republican" shield designed to give the Democratic Johnson political protection against Republican election charges in the 1968 presidential election. Due to its immense cost and strategic importance, ABM systems have often been the subject of low-key but intensely bitter partisan struggles.
Why did the Soviets and Americans accept this limitation? Nuclear strategists in the United States believed that allowing either country to develop a first-strike capability would be destabilizing and increase the likelihood of the use of nuclear weapons in a crisis. Soviet leaders suspected that the United States, with its mammoth resources and technological superiority, might well be able to create a leakproof defense. By limiting ABM systems to a marginal role, strategic stability would be assured (this is the logic better known as mutually assured destruction). In what appears to nonexperts to be ironic, this limitation of defensive arms led to treaties limiting the construction of offensive arms known as the SALT I treaties.
Conservatives in the United States, suspicious of the foreign policy machinations of Henry Kissinger and dubious of the Soviet-American detente engineered by President Richard Nixon, never accepted the logic of the ABM Treaty, which was designed and ratified under Nixon and Kissinger. Over the next decade, activists pressured Republican leaders to overturn the ABM Treaty and begin the construction of a massive anti-Soviet defense.
Nothing was deployed operationally until Patriot antiaircraft missiles were used in the 1991 Gulf War to attempt to intercept Iraqi Scud missiles. Post-war anlayses show that the Patriot was largely ineffective because of the limited range of its radar and the control system's inability to discriminate payloads from other objects when the Scud missiles broke up (or were broken up -- it's not clear which) during reentry. On the other hand, the Scud itself was highly inaccurate and not very reliable. It was more a psychological than real threat to military targets. Independent experts have concluded that the Patriot (originally designed, like the early Nike systems, as an anti-aircraft system) may not have hit a single Scud.
Testing of ABMs and ABM technology continued through the 1990s with mixed success. Use of non-nuclear interceptors requires that the interceptor physically contact the incoming payload -- a much more difficult problem. There are also many unresolved issues with warhead discrimination and decoy deployment. There is little doubt that occasional intercepts are possible. The issue is whether an ABM system is a cost effective deterrent or whether a potential enemy will simply deploy a few more missiles with more warheads.
Bush and his advisors appear to be determined to deploy a system whether it will work or not and have proposed to develop a dual purpose test and interception facility in Alaska. The Alaska site, it should be mentioned, might be effective against missiles launched from East Asia, but is not likely to provide much protection from missiles launched from Southwest Asia (Iran or Iraq, for example). Bush has used the September 11, 2001 Terrorist Attacks to justify the need for such a shield. This is despite the fact that a missile shield would not have protected the nation from that attack, which was not launched via missiles (and would not protect the U.S. from any future attacks which might choose to simply bypass the missile shield).
Often overlooked in the ABM debate in the United States is the resistance of many Pentagon leaders to the construction of a National Missile Defense. Admirals and generals of all services oppose spending huge sums (currently $8bn/yr in 2003) to research, develop, and procure NMD systems. They would prefer to have that money spent on new conventional weapons, training, equipment, or pay.