In a comment below, Nate sums up his position (more fully expressed here) about the Church's teaching on war:
My argument is simple - that the Church's thought has always been: "war is a necessary evil."
Now the Church's thought it, "war is an unnecessary evil."
As I replied at the time, that's a neat-- in the sense of tidy and orderly -- way of putting it. But we need to use this formula with care.
The term "necessary evil" is particularly tricky. To my mind, in common usage it connotes a certain fatalism, a degree of willing acceptance of moral evil in this vale of tears. The sort of pragmatism that doesn't look too closely at the means to a good end has no place in the Church's thought.
If we want to use "necessary evil" in a stricter sense, we immediately run into the problem Steven Riddle noted: that evil is never necessary. In fact, if by "evil" we mean "moral evil," the term "necessary evil" is an oxymoron, a logical impossibility that can be employed only if we don't really know what moral evil is.
That leaves us with the idea of "necessary material evil," where by "material evil" I mean simply a lack of something that shouldn't be lacking. Material evils are things like blindness and hunger and sickness. (They don't need to be physical; my failure to love you as I love myself produces the material evil of you lacking what my charity ought to provide.)
The Church does speak in terms of necessary material evil -- okay, not literally; I haven't seen the term "necessary material evil" used in teaching documents (or anywhere else, come to that). But, for example, the principle of double effect is basically one of necessary material evil: I take bad tasting medicine, not for the sake of the bad taste, but to be restored to health.
And of course "necessary" here means "unavoidable if the end is to be achieved," not "essential" or "required" in an absolute sense.
With that in mind, in what sense has the Church's thought always been "war is a necessary evil" and is now "war is an unnecessary evil"?