On March 23, 1983, the Strategic Defense Initiative (SDI) was proposed by US President Ronald Reagan, the 40th President of the United States, serving from 1981 to 1989. It is to use ground- and space-based systems to protect the United States from attack by strategic nuclear ballistic missiles, an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion, that follows a suborbital ballistic flight path with the objective of delivering one or more warheads to a predetermined target.