The Strategic Defense Initiative (SDI) is a system proposed by Ronald Reagan on March 23, 1983 to use space-based systems to protect the US from attack by strategic nuclear missiles. It is often called Star Wars, by which opponents imply it is an impractical science fiction fantasy, but supporters have adopted the usage as well on the grounds that yesterday's science fiction is often tomorrow's engineering.
The proposal was sharply criticized for its potential costs, doubts that it would be technologically feasible and afford complete protection against all delivery systems, concerns that it would violate the 1972 anti-ballistic missile treaty and destabilize the nuclear balance of power.
The research was controlled by the Strategic Defense Initiative Office, an agency of the DoD, until 1993 when it was renamed the Ballistic Missile Defense Organization. Up to 1990 $17bn was spent on research.
The project was repeatedly scaled back, renamed Global Protection Against Limited Strikes (GPALS) in 1991, dubbed "Son of Star Wars", and refocused on protection from smaller attacks. Based on a ground system of rocket-launched interceptors aimed a preventing small scale launches by "rogue states" (apparently North Korea, Libya, Iraq or Iran) from impacting on the US. Bill Clinton almost halted the program but it has regained momentum under George W. Bush. Apparently successful tests of the interceptors have been revealed as flawed or fixed. But the system is still intended to be deployed at a cost of up to $60bn by 2005. It is opposed by Russia as a violation of the ABM treaty and by most members of NATO.