There used to be a time when the threat of an ICBM was the worst thing ever (it still is but just not as bad in terms of helplessness). If a launch was detected there was not really anything you could do to protect yourself except pray that something goes horribly wrong with the missile and it just blows up in mid air.
In the last 50 years or so development in the anti ballistic missile systems has progressed a lot. From the unreliable systems that were produced in the 1960s we have come a long way. Here I present to you, How to bring down a ballistic missile. Interception is divided according to the phase in which the missile can be destroyed.
Boost Phase
The boost phase is when the rockets of the missile are firing. Due to the hot exhaust gases being emitted by the rocket it can be detected and targeted easily, further more decoys are also useless in this phase. For all its advantages there are a few draw backs as well. Firstly that to intercept the missile one usually needs to be close to the hostile territory doing the firing and secondly there is not a lot of time in which to target and destroy the missile, usually only 3 to 5 minutes.
The first line of defense in the boost phase is the Airborne laser being developed by Boeing. The laser which is mounted on a Boeing 747-400F has a high power laser mounted in its nose. It heats the skin of the target. The extreme heat generated due to this leads to the missile failing under stress from the high speeds encountered during flight. Flight tests are supposed to begin in this year, they will determine the fate of the project. The exact details of operation can be found here on wiki and in the video below.. The second line of defence during the boost phase is a kinetic energy interceptor. This is basically a mobile platform launched missile which will ram into the target and cause it to explode during the boost phase. development of this is still under way.
Mid-course Phase
The missile has successfully entered space and is now heading towards its target. You have around 20 minutes to bring it down. This is usually the preferred phase for destruction of the incoming missile since there is plenty of time to respond.
The missile can be shot down Ground based interceptors or naval launched interceptors. The Aegis ballistic missile defense system can bring down ICBMs in mid course phase or short range ballistic missiles in terminal phase. The radar on the aegis outfitted ships can also track ICBMs and guide ground based interceptors to destroy them. The interceptors are GPS guided. The interceptor works in the way that it enters space, the booster rockets fall off leaving a small craft to finish the job. The craft uses thrusters to steer and slams into the target. Kind of like hitting a bullet with a bullet.
Terminal Phase
This is the last line of defense. The missile can be destroyed in 30 seconds to a minute! With the missile heading towards its target with speeds as high as 10,000 miles per hour this is a nightmare scenario. This means that the time to react is very short and also that in case the ICBM is carrying a nuclear payload, it might be spread across the target area if the missile is destroyed. On the plus side smaller and lighter anti ballistic missiles will do the required job and less sophisticated radar (as compared to the mid course phase) are needed. The Patriot missile system is used to meet the challenge. Initially designed to destroy aircraft it has been upgraded to handle ballistic missiles. The on board radar on the patriot missile tracks the targets and slams into it. It only has a range of around 124 miles though.