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National missile defense (NMD) is a generic term for a type of missile defense intended to shield an entire country against incoming missiles, such as intercontinental ballistic missile (ICBMs) or other ballistic missiles. This is also used to refer to the American nationwide antimissile program the United States has had in development 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. ==Definitions== The term "national missile defense" has several meanings: * ''Most common, but now deprecated'': U.S. National Missile Defense, the limited ''ground-based'' nationwide antimissile system in development since the 1990s. In 2002 this system was renamed to Ground-Based Midcourse Defense (GMD), to differentiate it from other missile defense programs, such as space-based, sea-based, laser, robotic, or high-altitude intercept programs. As of 2006, this system is operational with limited capability. It is designed to intercept a small number of nuclear-armed ICBMs in the mid-course phase, using interceptor missiles launched from within the United States in Alaska. They use non-nuclear kinetic warheads. * Any national ICBM defense by any country, past or present. 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. The Russian A-135 anti-ballistic missile system is currently operational only around the city of Moscow, the national capital, and is far from being national in scope in Russia. * Any national missile defense (against any missile type) by any country. Israel currently has a national missile defense against short and medium-range missiles using their Arrow missile system. See Missile defense#Trajectory phase for the types of anti-ballistic missiles, the advantages and the disadvantages of each implementation type. The role of defense against nuclear missiles has been a heated military and political topic for several decades. (See also nuclear strategy, Missile Defense Agency, and anti-ballistic missile.) But missile defense against a known ballistic missile trajectory has to be rethought in the face of a maneuverable threat (such as a hypersonic glide vehicle, which still has yet to be realized and proven, as of 2015). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「United States national missile defense」の詳細全文を読む スポンサード リンク
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