Strategic Defense Initiative

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The Strategic Defense Initiative (SDI), commonly called Star Wars, was a proposal by U.S. President Ronald Reagan in 1983 to build a ground- and space-based system to protect the U.S. from nuclear ballistic missiles. The system was never fully set up, but the research from this paved the way for the anti-ballistic missile systems of today, including the Patriot missile that provided the defense against Iraqi Scud missiles launched against American forces and Israel during the Gulf War. It is believed by some experts that the Soviet's response necessitated by SDI contributed to their financial and political collapse.

SDI was a major component of the Reagan Doctrine in foreign policy, designed to push the Soviets to its limits and beyond into collapse.


Goals of SDI

The basic goal of SDI was to intercept and destroy incoming nuclear ballistic missiles to prevent detonation over the U.S. In some scenarios, not all warheads would be intercepted, but enough would be to render mutually assured destruction moot. In the scenario of "partial protection", the U.S. would be immune from complete destruction, which would deter the USSR from launching a first strike. Also see game theory.

Components of SDI

There are a limited number of ways to prevent a ballistic missile from detonating over one's territory. A brief primer on nuclear attack will help explain the problems to be solved.

The Problems

Missiles with nuclear payloads are launched in two primary ways: as ground based intercontinental ballistic missiles, and as shorter range missiles, such as submarine-launched ballistic missiles (SLBMs) and submarine-launched cruise missiles (SLCMs).

  • ICBMs are launched from the ground, travel into space, and return with the assistance of gravity. Most contain multiple independently targeted reentry vehicles or MIRVs. This means that a single ICBM can cause many nuclear warheads to detonate over multiple targets. Problems involved include detecting the launch, and deciding on what level to intercept the target. If the target is disabled before deployment of MIRVs, this obviously solves many problems. Once MIRVs are deployed, many more targets must be eliminated. Destroying an ICBM before it enters space is essentially impossible. Interception in space requires a pre-placed infrastructure. Interception after MIRV deployment involves ground-based systems.
  • SLBMs and SLCMs present serious difficulties. SLBMs have a shorter time available to intercept, as they can be launched nearer to target. SLCMs are basically a first-strike weapon, travel short distances quickly and close to the ground.

The Solutions

Ground-based systems

Extended Range Interceptor (ERINT)

Ground-based systems must be able to detect a rapidly-approaching ballistic object, and destroy or divert that object well enough to avoid catastrophic damage to the target.

Space-based systems

Space-based systems generally aim to destroy a ballistic missile at the height of its trajectory. At this point, the missile is fragile and vulnerable. The difficulty lies in detecting the object, deploying the countermeasure, and successful interception. Several solutions have been proposed, including "Brilliant Pebbles", which would be non-ballistic kinetic missiles deployed to intercept the missile, and at low energy, deflect or destroy the ICBM.[1]


Critiques of the original program were self-contradictory. One the one hand it was supposedly technologically impossible while simultaneously arguing that it effective and therefore was destabilizing.

The criticisms were substantially weakened by the successful missile defense deployed during the Gulf War, when the Patriot missiles successfully and spectacularly shot down deadly missiles launched by Saddam Hussein against military and civilian targets. Furthermore, contrary to popular opinion, relatively few false alarms were raised by automated missile defense systems during the tenure of SDI. The decade was a Renaissance of artificial intelligence and most experts in the field firmly stood behind Reagan's bold proposition that America could be defended from Communist aggression with a strategic defense program fully automated for optimal reaction time.


Under Reagan SDI explored advanced technologies with the aim of determining the feasibility of effective, nonnuclear ballistic missile defenses (BMD). The initial focus of SDI reflected US concerns about growing Soviet first-strike capabilities. Thus, an initial deployment of defenses, or Phase I Strategic Defense System, was to have provided the minimum defensive capability that would add meaningfully to deterrence of a Soviet first strike. However, with the end of the Cold War, President George H.W. Bush in early 1991 directed that SDI be refocused to provide global protection against limited strikes, thus shifting the primary US role for BMD from deterrence to actual protection.

See also

Further reading

  • Baucom, Donald R. The Origins of SDI, 1944-1983 (1992),
  • Westwick, Peter J. "'Space-Strike Weapons' and the Soviet Response to SDI," Diplomatic History 2008 32(5): 955-979


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