Missile defense is a branch of military science that focuses on neutralizing threats that come in the form of missiles. Originally, it focused on long-range nuclear missiles, but today a range of missiles are included in these programs. Many nations have some form of a missile defense program; countries that manufacture a range of missiles tend to have more extensive programs.
There are three basic kinds of defense against missiles. Strategic defense is designed to protect a country from long-range missiles, such as inter-continental ballistic missiles. Theater missile defense is focused on short-range missiles which could be used on the battlefield or in a theater of war, while tactical defense deals with extremely short-range tactical missiles.
Some people prefer to classify missile defense by the trajectory stage reached by potential threats; for example, it can focus on the booster stage, the mid-course phase, or the terminal phase. Other defense programs are geared towards specific points of interception, such as inside or outside the Earth's atmosphere.
Several goals are addressed by defense programs around the world. The first is the ability to identify incoming missiles and classify them to determine how much of a threat they pose. Next is the ability to track missiles, with the theoretical capability of predicting their path. Interception and destruction are also critical, as it doesn't do a country much good to know that a missile is coming if it can't do anything about it.
Since the 1940s, when several nations began producing missiles and recognizing that these weapons could be potentially devastating, the field of missile defense has expanded radically. Some programs have been heavily criticized because they tend to eat up a great deal of military spending, often with minimal and sometimes disappointing results. A number programs have even ended up as total failures, much to the disappointment of their sponsors.
Missile defense is an interesting example of the catch-22 many militaries find themselves in. Because missiles have been developed as a military weapon, it is necessary to come up with ways to defend countries from missile attacks. The development of programs to defend against missiles has led many nations to come up with more innovative weapons that are designed to subvert existing missile defense systems, thereby feeding a need for better defense, which forces the development of better missiles, and so forth.