The Strategic Defense Initiative (SDI) was a proposed missile defense system intended to protect the United States from attack by ballistic strategic nuclear weapons (Intercontinental ballistic missiles and Submarine-launched ballistic missiles). The system, which was to combine ground-based units and orbital deployment platforms, was first publicly announced by President Ronald Reagan on March 23, 1983. The initiative focused on strategic defense rather than the prior strategic offense doctrine of mutual assured destruction (MAD). The Strategic Defense Initiative Organization (SDIO) was set up in 1984 within the United States Department of Defense to oversee the Strategic Defense Initiative.