National missile defense (NMD) has a number of meanings. First and foremost, it is the concept of a comprehensive missile defense system for a country, to protect it against incoming missiles, especially nuclear warhead-armed intercontinental ballistic missiles (ICBMs). The term used to refer to U.S. National Missile Defense, a ground-based missile defense shield that was renamed Ground-Based Midcourse Defense in 2002. Now it most frequently refers to the overall limited U.S. nationwide antimissile program in development since the 1990s. In contrast to the failed Strategic Defense Initiative system, designed to intercept a full-on nuclear attack by the Soviet Union, which was championed by Ronald Reagan in the late 1970s and early 1980s, the present-day Nation Missile Defense system is only designed to intercept a limited number of ICBMs sent by a smaller adversary.
The mechanism of national missile defense, and the one currently installed for the United States, is the use of fast interceptor missiles to strike down incoming ICBMs before they detonate. Such a missile system would likely be too slow to react to submarine-launched ballistic missiles, which could reach their targets in less than a minute after a full-scale nuclear war broke out, but it could be quick enough to respond to ICBMs, which would not reach their targets until 15-20 minutes. The current national missile defense system is the latest incarnation of a long series of (mostly failed) systems under development: Project Nike (late 1950s), Project Defender (early 1960s, never deployed), the Sentinel Program (early 1960s, never deployed), the Safeguard Program (late 1960s, never deployed), and the Strategic Defense Initiative (1980s, never deployed).
Abandoning the ambitions of the past to provide a missile shield effective during an all-out attack, the present-day National Missile Defense takes a more pragmatic approach: protecting the United States from nuclear blackmail or a limited attack by a rogue state like a future North Korea or Iran. As of 2009, ten missile interceptors are deployed, along with a sophisticated radar facility in Alaska, a site that was chosen due to possible attacks from North Korea. The deployment of the system is envisioned in three phases, and of 2009, the first phase is not yet complete. The first phase would be to defend the United States against a few dozen missiles without significant countermeasures. Upgrades to functionality would follow in the late 2010s. Tests of the system carried out in 2002 were successful, and viewed by most independent commentators as legitimate, in contrast to earlier tests that received accusations of rigging.
