Fear
I have a little problem with the argument "imagine what might have happened if we HADN'T gone to war".
We saw here the power of imagination to come up with a wide range of nightmare scenarios, but yet they're not all likely, are they? And perhaps none of them will come to pass. Should we act on our imagination nonetheless?
The same could be said of the use of imagination as a justification for preventative war--what we're really preventing isn't so much a certain future containing a concrete danger, but simply the possibility of the realization of our darkest fears. Should we be driven to momentous decisions by fear alone?
--Robert