I used to think of myself as a complete pacifist, but these words haven’t left my mind since I heard them:

You think you’re better than everyone else, but there you stand: the good man doing nothing. And while evil triumphs and your rigid pacifism crumbles into bloodstained dust, the only victory afforded to you is that you stuck true to your guns.

Of course this only applies to defense, never to offense (especially “preemptive defense”), but I can’t really argue against it.