Nuclear War b/w two Nuclear power's

Yes the whole idea of nuclear war is utterly absurd anyway. This is just one aspect of it! Why anyone would want to launch one is beyond me. I wish they had never been invented, but now they have they must never been used. The missile shield as proposed by USA is also absurd as you have suggested as yes there is only a brief time after launch when the missile is visible and no one knows what the launch is until it is too late. I think there are more important things to worry about as if a nuclear war starts it will all be over for you and me in an instant! Nothing we can do but enjoy our life now and try and make this world a better place so there is no need for any kind of war ever again!