Reagan's Strategic Defense Initiative (SDI) is a technically feasible missile defense system that could render nuclear weapons obsolete.
The American Physical Society's 1987 report concluded that SDI was at least a decade from even beginning meaningful tests. The technology Reagan described did not exist and no comprehensive missile defense system exists today.
What changed?
President Reagan's March 23, 1983 address had an almost cinematic ambition. Imagine, he invited the nation, a defense system capable of intercepting nuclear missiles before they reach American soil, making nuclear weapons themselves "impotent and obsolete." The press labeled it "Star Wars." In civics and government classrooms through the mid-1980s, SDI was typically taught as a serious defense program: a bold technological challenge the United States was rising to meet.
The scientific establishment was considerably more skeptical. In April 1987, the American Physical Society published a two-year study examining SDI's feasibility. The conclusion was blunt: the technologies required to build a space-based missile defense system did not yet exist, were not close to existing, and would take at least a decade simply to test in any meaningful way. Directed-energy weapons powerful enough to destroy missiles during their brief boost phase would require energy sources far beyond anything then available. Onboard computers of the era could not perform the target acquisition and discrimination tasks required within the decision windows a nuclear attack would allow.
What the classroom version of SDI often omitted was that the program was as much diplomatic tool as engineering project. At Reykjavik in October 1986, Reagan and Gorbachev came close to agreeing to eliminate all ballistic nuclear weapons before negotiations collapsed, specifically because Reagan refused to confine SDI to the laboratory. Gorbachev feared a working system could neutralize Soviet deterrence; Reagan believed in the dream too completely to trade it away. Whether SDI's value was strategic fiction or sincere vision, it was doing real work in the Cold War, just not the technical work its advocates described.
Decades of successor programs have produced the Ground-Based Midcourse Defense system, which has recorded mixed results in controlled tests against single, unsophisticated warheads under favorable conditions. A comprehensive shield against a peer nuclear arsenal remains on no credible technical horizon. The vision Reagan described in 1983 was genuinely inspiring. The physics remained uncooperative.