Missile defense systems use high-precision radar to track incoming missiles and direct defensive missiles to impact and destroy them. In the case of ballistic missile defense, interceptors attempt to strike incoming weapons during their boost phase or in orbit, before they reach a velocity that would make them impossible to hit. Interceptors attempt to either strike the incoming missile or detonate close enough to it to disrupt the weapon.
Missile defense systems were first developed at the dawn of the nuclear age and were a point of contention between the United States and the Soviet Union. If either side managed to create a missile shield impervious to their opponent's intercontinental ballistic missiles, it would render the protected country safe from the mutually assured destruction that prevented actual nuclear war. For this reason, treaties between the two superpowers limited anti-missile systems as well as the missiles themselves.
The first documented use of a missile defense system occurred during the Gulf War when Patriot anti-missile batteries intercepted Scud missiles fired by Iraq. Short-range missile defense systems like the Patriot are effective against short-range rocket attacks but cannot target ballistic vehicles due to the high speeds involved during the re-entry phase.
The most infamous missile defense system was the Strategic Defense Initiative, or "Star Wars," proposed by President Ronald Reagan in 1983. It was to serve as a global missile shield against ICBM attack and caused massive concern in the USSR. However, it wasn't achievable with the technology of the time, and the plan was scrapped in favor of a more directed missile defense system in the 1990s.