United States national missile defense

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.

The term "national missile defense" has several meanings:

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).

When the United States Air Force was split from the United States Army in 1947, the Army retained the role of ground based air defenses that would evolve into National missile defense. The Army retained the lead role in this area until the success of the Aegis system shifted the focus to the United States Navy in the 21st century.

...
Wikipedia