Strategic Defense Initiative

US programme (1983–93) to explore the technical feasibility of developing a comprehensive defence system against incoming nuclear missiles, based in part outside the Earth's atmosphere. The programme was started by President Ronald Reagan in March 1983, and was overseen by the Strategic Defense Initiative Organization (SDIO). In May 1993, the SDIO changed its name to the Ballistic Missile Defense Organization (BMDO), to reflect its focus on defence against short‐range rather than long‐range missiles. SDI lives on today in the less ambitious National Missile Defense (NMD) programme.

The aim of SDI was to create a ‘defence shield’ that would protect the USA from a full nuclear missile attack by the Soviet Union or other hostile nuclear powers. This would end reliance on offense‐dominated deterrence (‘Mutually Assured Destruction’) through a balance of terror. Enemy missiles would be attacked at several different stages of their trajectory, using advanced laser and particle‐beam technology, thus increasing the chances of disabling them.

The programme drew support from Republicans, but faced strong opposition on technical, economic, and diplomatic grounds. Some scientists maintained that SDI was unworkable, as a 100% strike rate against incoming missiles would be impossible to achieve. In 1988 the Joint Chiefs of Staff announced that they expected to be able to intercept no more than 30% of incoming missiles. SDI was criticized on cost grounds, being both hugely expensive – by 1993 US$25 billion had been spent on SDI research – and vulnerable to cheaper counter‐measures designed by missile developers to confuse the missiles's interceptors. Diplomatically, Russia and China both fiercely opposed SDI, and the deployment of such a system would have been a breach of the 1972 Anti‐Ballistic Missiles (ABM) treaty. However, SDI's supporters believed it to be a valuable bargaining chip in arms reduction negotations with Russia, which (despite attempting to develop its own version of SDI) realized it could not match the USA in a race in this high technology area.

SDI research led to development of ‘hit‐to‐kill’ (HTK) technology and miniaturization of rocket components. Israel, Japan, and the UK were among the nations that assisted in SDI research and development. The ending of the Cold War in 1990 led to reduced US–Russian tensions, which led to a US decision, in 1991, to scale down SDI.

However, a new threat emerged: the spread of ballistic missiles and weapons of mass destruction to smaller states such as North Korea, Iran, and Iraq. A new approach of Global Protection Against Limited Strikes (GPALS) was adopted by President George Bush, with the goal of defending the USA against limited missile attacks, and protect deployed US forces and allies against shorter‐range ballistic missiles. It was based on three components: a global, space‐based system of Brilliant Pebbles interceptors; the ground‐ and sea‐based Theatre Missile Defense; and a limited, ground‐based national missile defence element.