text: The Strategic Defense Initiative ( SDI ) was proposed by U.S. President Ronald Reagan on March 23 , 1983 to use ground and space-based systems to protect the United States from attack by strategic nuclear ballistic missiles .
simplified: The Strategic Defense Initiative ( SDI ) was a plan created by U.S. President Ronald Reagan on March 23 , 1983 to use ground and space-based systems to protect the United States from attack by nuclear ballistic missiles .