http://en.wikipedia.org/wiki/Dead_Hand_(nuclear_war)
Its purpose was to actually two fold -- ensure the ability to launch a retaliatory attack after a "surgical" nuclear strike and (and this follows from the first) ensure that hot headed generals or future leadership wouldn't be too quick to press the trigger on the very first warning (which might be a false alarm, and these have happened in the past).
That is a pretty nice system come to think about it.
So on first ever threat the #1 thing to do is enable this system. Then decide what to do, knowing, it would be possible to retaliate.
Note, what is interesting, they didn't broadcast to the world that they had it. Which is interesting. From a game-theoretic point of view, shouldn't they have broadcasted its existence as that would have discouraged a first strike?