Just think of all the years and billions of dollars spent on "national defense", and this country is still vulnerable to biological attacks and poisonings. An airliner was able to crash into the Pentagon, how much more efficient would a first strike be???
It doesn't matter how many billions of dollars you throw at defense. It's impossible to forsee every possible threat made by every person who wants to do others harm. It's compounded even worse when you are a "free" society.