SDI, or Strategic Defense Initiative, was a proposed missile defense system announced by U.S. President Ronald Reagan in 1983. The initiative aimed to protect the United States from nuclear missile attacks, particularly from the Soviet Union, by using ground-based and space-based systems to intercept and destroy incoming missiles. The program focused on advanced technologies, including lasers and satellite systems, to create a protective shield. While it generated significant debate and controversy regarding its feasibility and cost, SDI ultimately influenced discussions on missile defense and arms control during the late Cold War era.