United States National Missile defense
The U.S. Sentinel program was a planned national missile defense during the 1960s, but was never deployed. Elements of Sentinel were actually deployed briefly as the Safeguard Program, although it wasn't national in scope. United States has had in development a nationwide antimissile program since the 1990s. After the renaming in 2002, the term now refers to the entire program, not just the ground-based interceptors and associated facilities. This article focuses mainly on this system and a brief history of earlier systems which led to it.
Other elements yet to be integrated into NMD may include anti-ballistic missiles, or sea-based, space-based, laser, and high altitude missile systems. The NMD program is limited in scope and designed to counter a relatively small ICBM attack from a less sophisticated adversary. Unlike the earlier Strategic Defense Initiative program, it is not designed to be a robust shield against a large attack from a technically sophisticated adversary.
As of 2012, this system is operational with limited capability.