Missile defense is a catch phrase to encourage public support for an enormously expensive offensive nuclear first strike policy.
It’s a bullshit term just like “death tax” or “partial birth abortion”.
It was conceived during a time where full scale use of nuclear weapons by either major power would ensure the destruction of BOTH countries. It assumes that each side has enough weaponry to destroy the other side and that either side, if attacked for any reason by the other, would retaliate with equal or greater force. The expected result is an immediate escalation resulting in both combatants' total and assured destruction.
Take for example if the U.S. launched a first strike on the soviet union, the soviets would have more than enough time to either launch their missiles before the U.S. missiles hit their target or they would have enough missiles left over to fire after U.S. missiles landed to completely destroy the entire U.S. Missile “defense” was conceived as part of a first strike strategy, after you eliminate the majority of the soviet union with your first strike the remaining soviet missiles, at least some, could be shot down before they reached the U.S.
It was never conceived as a defensive system because given the huge build up of arms on both sides it would be impossible for the system to shoot down every missile. It was a way to make sure your dying enemy couldn’t get off a final shot and take you down with them.
That’s why it has been so controversial since it was first proposed, because it eliminated the effectiveness of Mutually Assured Destruction(MAD) under which neither side would dare to launch a first strike because the other side will launch on warning (also called fail-deadly) or with secondary forces (second strike) resulting in the destruction of both parties.
Some of you may remember what it used to be called before they came up with the catchy name of missile defense . . . Strategic Defense Initiative (known as "Star Wars")
SDI was criticized (including Margaret Thatcher) because, were it ever operational and effective, it would have undermined the "assured destruction" required for MAD. If America had a guarantee against Soviet nuclear responce, it would have first strike capability which would have been a politically and militarily destabilizing position.
To prevent a power from using first strike while having a “missile defese” system officially the world had the Anti-Ballistic Missile Treaty ( which George W. Bush withdrew from in June 2002).
it's so we can launch first, not if we are off guard