It is paradoxical that nation states which base their justification on the need to protect their citizens expose their citizens to the possibility of annihilation. If we forget about nationality for a moment and think in terms of people and governments, one could say that the governments of both the United States and the Soviet Union are endangering the citizens of both countries. On one side you have people who want to be left alone to lead their lives, and on the other side you have governments building armaments that make people less secure. There are more paradoxes in the notion that nation states can protect citizens in the nuclear age. It is paradoxical to speak of national defense when there is no defense against atomic weapons.
Defense means the ability to ward off an attack by an enemy.
No one believes that it is possible to ward off a nuclear attack once one is launched against one's country, so it simply does not make sense to speak of national defense in the nuclear age. The best that the military planners hope for is deterrence. Their theory is that if they have enough weapons to destroy the other side, the other side will be deterred from attacking. Basically, that program is an attempt to provide a real defense for the United States by having devices that would intercept any incoming missiles. The Star Wars program would definitely start a new arms race and make the Soviet Union fear a first strike because it would appear that the United States was preparing to survive a nuclear war.