Richard Fernandez explains that nuclear deterrence depends heavily on sowing uncertainty in the minds of your enemies, and that the American missile shield we're installing in Eastern Europe is a key contributor to that uncertainty.
What a working missile defense shield will do is make any Russian limited WMD attack on the West a very uncertain proposition. While Russia’s arsenal is easily big enough to overwhelm, through sheer numbers, the defensive system based in Poland and the Czech Republic any such attack would also be big enough to guarantee Russia’s destruction in the resulting retaliation. It may be an exaggeration to claim that a missile defense will have the effect of disarming the Kremlin of any viable military response between issuing a diplomatic protest and starting Armaggedon but it is quite clear it threatens to invalidate a large range of the “full spectrum” responses now available to the Russians.
He also quotes from "Essentials of Post-Cold War Deterrence" which explains that deterrence doctrines have never depended on so-called "rational" enemies.
Deterrence of the Soviets never depended on having “rational” leaders. Stalin was in charge when the Soviets first began a build-up of nuclear arms, and it is difficult to consider him as an example of a rational leader. This is perhaps the grossest error of those who make arguments that the new multilateral threats are “undeterrable” because the new regional actors are not likely to be rational. Stalin was hardly more rational than they. The very framework of a concept that depends on instilling fear and uncertainty in the minds of opponents was never, nor can it be, strictly rational. Nor has it ever strictly required rational adversaries in order to function. What should be sobering to all of us in viewing deterrence as a process is that its outcome was never, nor can it ever be, strictly predictable. ...
Because of the value that comes from the ambiguity of what the US may do to an adversary if the acts we seek to deter are carried out, it hurts to portray ourselves as too fully rational and cool-headed. The fact that some elements may appear to be potentially “out of control” can be beneficial to creating and reinforcing fears and doubts in the minds of an adversary’s decision makers. ‘This essential sense of fear is the working force of deterrence. That the US may become irrational and vindictive if its vital interests are attacked should be part of the national persona we project to all adversaries.
An excellent and concise explanation of the psychology that helped us win the Cold War and continues to protect us today.