The Strategic Defense Initiative (SDI), derisively nicknamed the "Star Wars program", was a proposed missile defense system intended to protect the United States from attack by ballistic strategic nuclear weapons (intercontinental ballistic missiles and submarine-launched ballistic missiles). The concept was announced on March 23, 1983, by President Ronald Reagan, a vocal critic of the doctrine of mutually assured destruction (MAD), which he described as a "suicide pact". Reagan called upon American scientists and engineers to develop a system that would render nuclear weapons obsolete.

The Strategic Defense Initiative Organization (SDIO) was set up in 1984 within the US Department of Defense to oversee development. A wide array of advanced weapon concepts, including lasers, particle beam weapons and ground- and space-based missile systems were studied, along with various sensor, command and control, and high-performance computer systems that would be needed to control a system consisting of hundreds of combat centers and satellites spanning the entire globe and involved in a very short battle. The United States holds a significant advantage in the field of comprehensive advanced missile defense systems through decades of extensive research and testing; a number of these concepts and obtained technologies and insights were transferred to subsequent programs.

Under the SDIO's Innovative Sciences and Technology Office, headed by physicist and engineer Dr. James Ionson, the investment was predominantly made in basic research at national laboratories, universities, and in industry; these programs have continued to be key sources of funding for top research scientists in the fields of high-energy physics, supercomputing/computation, advanced materials, and many other critical science and engineering disciplines and funding which indirectly supports other research work by top scientists.

In 1987, the American Physical Society concluded that the technologies being considered were decades away from being ready for use, and at least another decade of research was required to know whether such a system was even possible. After the publication of the APS report, SDI's budget was repeatedly cut. By the late 1980s, the effort had been re-focused on the "Brilliant Pebbles" concept using small orbiting missiles not unlike a conventional air-to-air missile, which was expected to be much less expensive to develop and deploy.

SDI was controversial in some sectors, and was criticized for threatening to destabilize the MAD-approach potentially rendering the Soviet nuclear arsenal useless and to possibly re-ignite "an offensive arms race". Through declassified papers of American intelligence agencies the wider implications and effects of the program were examined and revealed that due to the potential neutralization of its arsenal and resulting loss of a balancing power factor, SDI was a cause of grave concern for the Soviet Union and her primary successor state Russia. By the early 1990s, with the Cold War ending and nuclear arsenals being rapidly reduced, political support for SDI collapsed. SDI officially ended in 1993, when the Clinton Administration redirected the efforts towards theatre ballistic missiles and renamed the agency the Ballistic Missile Defense Organization (BMDO).

In 2019, space-based interceptor development resumed for the first time in 25 years with President Trump's signing of the National Defense Authorization Act. The program is currently managed by the Space Development Agency (SDA) as part of the new National Defense Space Architecture (NDSA) envisioned by Michael D. Griffin. Early development contracts were awarded to L3Harris and SpaceX. CIA Director Mike Pompeo called for additional funding to achieve a full-fledged “Strategic Defense Initiative for our time, the SDI II".