Unlike the other top weapons of the Cold War, the SDI was more of a research and development program than practical weapon. President Ronald Reagan proposed the SDI in 1983, suggesting that American should defend itself from Soviet ICBMs by placing high-tech defenses in space.
The strategy spurred criticism by Reagan's political opponents and the media, who dubbed the program "Star Wars." The Soviets paid more respectful attention, especially since the SDI concept threatened to overturn the MAD Doctrine. In theory, if SDI worked America could defend itself from Soviet missiles, dramatically reducing the USSR's nuclear threat.
While the full vision of SDI was never realized, the program showcased America's technological edge over the Soviets. Some analysts note the Soviet Union increased its military spending, at the expense of consumer products, during this period. They assert (against contrarian voices) that the SDI's impact on Soviet budgets and strategy helped contribute to the collapse of the USSR.
Elements of the SDI have flowed into America's current anti-ballistic missile programs, which are now focused more on rogue states such as North Korea and Iran.