The Strategic Defense Initiative (or SDI) was a program creating a network of land and space-based anti-missile defenses on Earth, intended to neutralize any international offensive missile attacks. It was initially proposed in 1983, at a very tense point in the Cold War, by United States President Ronald Reagan, and was given the nickname "Star Wars", after a popular film series of the day.
When Reagan and Soviet General Secretary Mikhail Gorbachev met in Reykjavik, Iceland in 1986, Gorbachev made it clear that he didn't want the SDI deployed and would be willing to make a significant reduction in Soviet nuclear weapons in exchange for not deploying it. (TOS novel: The Rise and Fall of Khan Noonien Singh, Volume 1)
The SDI network was deactivated with the signing of the United Earth Accords. It was the last space-based military system directed at Earth, as opposed to extraterrestrial threats. (TOS novel: Strangers from the Sky)
The SDI concept may have served as an inspiration for the Mars Defense Perimeter, which was designed with a similar purpose of neutralizing and deterring offensive actions against the inner Sol System. (TNG episode: "The Best of Both Worlds, Part II")
- SDI has not been fully implemented as of this date (the threat it was intended to counter would cease to exist within a decade), and has gone through several name and mission objectives changes since the publication of Strangers.