There have recently been new discussions, started by George W. Bush's administration and Secretary of Defense Donald H. Rumsfeld, about national missile defense (NMD). In view of this, here is where things now stand.

Current types of missile defense systems:

The lower tier system is designed for short-range (under 600 mi.) ballistic missile attacks. This category includes (1) PAC-3 and (2) Navy Area. PAC-3 stands for Patriot Advanced Capability 3, a surface to air missile used to defend against short ranged attacks. The PAC-2 was used in the Gulf War to destroy Iraqi scud missiles. The PAC-3 is intended for the protection of allies, rather than attacks on United States territory. The Navy Area missile defense is a sea-based missile defense capability utilizing Aegis destroyer ships with radar systems to detect and track sea-based missiles. A modified surface to air missile is also used here.

The upper tier involves defending attacks from medium range (beyond 600 mi.) ballistic missiles. (1) THAAD, Theater High Altitude Area Defense, with the purpose of protecting deployed troops, allies and friends. A ground-based missile defense system, it is specifically designed to defend against Theater Ballistic Missiles (TBMs). (2) The Navy also has a theater system for medium range missiles.

Proposed missile defense plans would focus on attacks on the U.S. by larger, longer-range missiles, specifically intercontinental ballistic missiles (ICBMs). A missile shield, as the G.W. Bush administration has stated an intention to develop, is controversial because of the 1972 Antiballistic Missile Treaty (ABM treaty) between the U.S. and the Soviet Union. The treaty prohibits both countries from developing and deploying such a national missle protection. It was a cornerstone of arms control during the Cold War, as it ensured the stability of Mutually Assured Destruction (MAD), an integral part of nuclear deterrence theory. Those in favor of developing a missile shield claim that the ABM treaty is no longer valid since the breakup of the Soviet Union in 1991. However, critics bring up the point that after Boris Yeltsin was elected president, he declared that Russia would continue to uphold all treaties previously signed by the Soviet government.

thanks go out to Jurph for feedback regarding the fickle nature of missile defenses :)