Strategic Defense Initiative (SDI), U.S. government program responsible for research and development of a space-based system to defend the nation from attack by strategic ballistic missiles (see guided missile). The program is now administered by the Missile Defense Agency (originally the Strategic Defense Initiative Office), a separate agency in the U.S. Dept. of Defense. SDI, popularly referred to as "Star Wars," was announced by President Ronald Reagan in a speech in Mar., 1983, and was derided by his critics as unrealistic. Space programs from other agencies and services were brought together in the organization. It has investigated many new technologies, including ground-based lasers, space-based lasers, and automated space vehicles. Critics argued that the original SDI program would encourage the militarization of space and destabilize the nuclear balance of power, and was technologically infeasible, based on untested technologies, and unable to defend against cruise missiles, airplanes, or several other possible delivery systems. In addition, some countermeasures to SDI technologies, such as decoy missiles and shielding of armed missiles, would be simple to implement. In 1987 the Soviet Union revealed it had a similar program.

The end of the cold war led to criticism that SDI was unnecessary, and in 1991 President G. H. W. Bush called for a more limited version using rocket-launched interceptors based on the ground at a single site. In 1993, SDI was reorganized as the Ballistic Missile Defense Organization (BMDO). The more limited system, called the National Missile Defense (NMD), is intended to protect all 50 states from a rogue missile attack, but the deployment of such a system was forbidden under the 1972 Anti-Ballistic Missile (ABM) Treaty. Russia opposed the NMD plan but, under President Putin, also proposed a mobile, pan-European missile defense system with a similar purpose that would not violate the ABM treaty. In 2001, President George W. Bush called for accelerated development of the NMD system, and subsequently withdrew from the ABM treaty to permit the system's development and deployment. Apparently successful early tests of the U.S. system were later revealed to have occurred after the odds of success had been enhanced (1984, 1991). Subsequent tests were generally more successful, although flawed or limited in certain respects, but tests in 2002, 2004, and 2005 involved failures. In 2002, President Bush ordered the deployment of a modest missile defense system by 2004, with interceptors based at sea and at Fort Greely, Alaska, and Vandenberg Air Force Base, Calif., and several interceptor missiles were emplaced by the end of 2004. Also in 2002, the BMDO was renamed the Missile Defense Agency (MDA). In addition to NMD, the MDA is also working to develop missile defenses for the battlefield as part of the Theater Missile Defense program.