[PvK]

Well, during the Cold War, and perhaps still today, an anti-missile shield is also a threat. Imagine you are a NATO commander during a tense part of the Cold War. The USSR starts deploying a defensive system that will remove your ability to hit the USSR with ballistic missiles, but of course it won't remove their ability to use theirs. It works both ways. Of course, there are huge obstacles to actually achieving a real feeling of safety when the opponent is as over-armed as the USA or USSR with MIRV ICBMs, cruise missiles, etc. Then there are the considerations that people can sneak weapons around on the ground (one supposed current concern), and for a nuclear superpower, the fact that they can still achieve Mutual Assured Destruction merely by threatening to detonate their arsenal even in their home country, since the environmental effects would destroy the world anyway.

As for the original laser questions, mirrors reflect some percentage of the light, and a certain amount of energy is required to melt the mirrored surface and distort it so it stops reflecting, and then a different amount of energy is required to burn through the surface beneath the mirror. So there's both a dividing factor and a threshold. Late 1980's discussions included ideas such as rotating, mirrored ICBM's. Eseentially though, like much of the Reagan/Bush/Bush military operations, the Star Wars program was more of a "look how scary nuts we are" and "let's dump tons of money into our supporters' corportations" plan, than really a "let's be Buck Rogers and really protect the Free World (tm) from the Evil Empire" plan.

PvK