In 1968, the threat of a Soviet or Chinese nuclear attack drove the United States to try to develop a national missile defense shield. However, it would be easy to elude this early, primitive missile shield. This and the fear that a missile defense shield would trigger a new arms race with the Soviet Union led to the signing of the Anti-Ballistic Missile (ABM) Treaty in 1972. This treaty prohibits building a nation-wide missile defense system and from deploying weapons in space.

More than 30 years later, the US is still without a missile defense system. The US won the cold war, and the threat of a Soviet nuclear strike has ceased to exit. However, ballistic missile technology is rapidly proliferating, with nations hostile to the US such as North Korea gaining the technology to build long-range nuclear missiles. The US is [or says that it is] concerned that a "rouge nation" (meaning Iraq, Iran, or North Korea) would eventually aquire the ability to strike the US with an intercontinental ballistic missile (ICBM). Accidental Russian and Chinese launches are also considered a threat.

In addition, the technology required to build a missile defense shield has advanced considerably. Better computers and more advanced sensors can make hitting a missile easier, and will not require the nuclear-tipped interceptors envisioned in the 1960s. Instead, America could use more politically acceptable "hit-to-kill" missiles, which destroy their targets by direct high-speed collisions.

Proponents of missile defense argue that the greater threat and more advanced technology make the development of a missile defense shield vital to our national security.

Presently, only one country hostile to the US, North Korea, possesses missiles that could reach any part of the US, and that missile (the Taepo-Dong 2) is currently untested. However, a recent study by former Secretary of Defense Donald Rumsfield (who is of course very biased towards developing a missile defense system and has lobbied for creating one in the past) concluded that a "rouge nation" could develop an ICBM within five years of deciding to do so and that the "very real" threat of ICBM attack could occur with "little or no warning". This contradicted a previous report by the CIA, which stated that it was not necessary to rapidly deploy a missile defense shield.

The threat of an attack by "rogue" nations like North Korea is the primary reason given for building a missile defense. The current Taepo-dong 1 missile can only carry a 1,000-kilogram nuclear bomb for about 2,500 kilometers, not enough to reach the US. It could also carry lighter biological or chemical weapons for 4,100 kilometers, but it would still be about 400 kilometers short of the two closest parts of America - the tip of Alaska and the Hawaii islands. However, the untested Taepo-dong 2 missile can just barely reach Alaska. Of course, North Korea could also launch a short-range nuclear missile from a ship, and the current missile defense system wouldn't be able to defend against it. In addition, if a nuclear bomb was to be launched from a ship, it would be more difficult to track where it came from, and the US probably wouldn't be able to retaliate.