In a National Review column, Victor Davis Hanson (I wrote a small piece of fanmail here) brings his wonderful gift of prose and insight to a discussion on the morals of preemption:

The truth is that one can sound moral only through the advocacy of restraint, never preemption. Appeasement wins applause for its ethical posturing and non-belligerency; and even when the corpses later pile up it rarely earns the disgust it deserves for getting thousands killed. In contrast, preemption is always equated with blood lust; and even when it saves thousands, critics sigh that in retrospect there must have been a better way.

The anti-preemption moralists pass simplistic judgement by saying "the ends NEVER justify the means".  Hence, the ease with which they're able to make moral equivalence statements like "the US has killed as many civilians in Afghanistan as the Terrorists killed on 9/11"  (which, BTW, is a highly refuted factoid).  Because, in their belief, the means can never be justified, all those using the means are equally criminal.

And, it wouldn't be a Hanson article with a few historical references tossed in.   In this case, I'll pull in one from recent history:

Like communism and socialism, the rhetoric of appeasement focuses on the pretense of human kindness and brotherhood, never on the calculus of the dead to come. Before preemption in Bosnia and Kosovo the world talked of tens of thousands of innocents murdered; after the bombing abated, it suddenly forgot the holocaust abated and an entire people saved, and instead turned its invective toward an interventionist U.S. that lost no American lives as its errant bombs killed a few hundred foreign civilians.