Original: 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.