Soon after the war ended states began to develop long-range missiles of their own, attempting to incorporate nuclear warheads in the missile. Developments for defense was more oriented towards aircraft than missile at this time. Early missile defense was crude and under funded, until October 4, 1947 when the Russians launched Sputnik on top of a multi-stage missile, later followed by the American's answer Explorer I. (2).
From the time a missile is launched to the time it lands, it goes through three phases. The first phase is called Boost Phase. This phase begins at launch continuing until the missile enters the atmosphere. Boost Phase is relatively short (3-5 min.), but proves to be the ideal phase for intercept. (5) If caught before entering the atmosphere, the missile will most likely explode over the country from which it was launched. This is important with chemical, biological, or nuclear weaponry, which could prove catastrophic if allowed to strike. Although ideal in principal, there are many problems that come with Boost Phase defense. First of all the information would have to be relayed, from the initial detectors to the interceptor itself, very accurately in a short amount of time. Secondly the interceptors would have to be relatively close, or extremely fast, to be able to reach the missile in time. (5).
The second phase, also the longest, is termed Midcourse Phase. (5) From the time it enters, to the time it exits the atmosphere the missile is considered to be in this phase. After the missile enters the atmosphere it is no longer being fueled giving it a more distinct route, thus being easier to predict. This is largest widow of opportunity for intercept. Also since there is particularly more time during this phase, less launch sites are needed to cover a larger area (5) The disadvantage to the extra time is that the perpetrator has the same amount of time to counter the interceptor.