1. War is always bad; it is an entire cascade of terrible consequences, and there is no war that is not. No matter how one acts, bad things happen. There is a tendency, however, to conflate the badness of war with the injustice of war; in reality the latter is only a small sub-region of the former. It's one thing to press for reducing the badness in the limited ways available for doing so, but you go wrong when you take bad happenings as themselves a sign of injustice.
2. Injustice in war is, as I noted, a small sub-region of the badness of war. The principles of just war require that one act in such a way as to render what is owed to everyone, even to enemies, but especially to relative innocents. It is not impossible to be just in warring; but it is very difficult, and requires a recognition that no atrocities of the foe can possibly justify the doing of evil in return. Yet there's also another aspect of justice in war that is often forgotten -- namely, a state going to war on behalf of its citizens has obligations of justice to its citizens to right the wrongs that occasioned the war. This is often the most difficult part of justice in matters of war, because it's important both to uphold the obligation and to prevent it from becoming an excuse for other injustices.
3. A word that people seem to like to throw around is 'war crimes'. But it's important to grasp that not every action with bad, even terribly bad, consequences in a war is a war crime; and, indeed, not every unjust action in war is a war crime. War crimes are a very narrow sub-region of unjust actions in war. You don't have a war crime without an applicable law. What is more, international law doesn't really work like ordinary civil and criminal law; international law is by its nature a matter of negotiation, and because of that, it both changes through time and only applies so far as you can make it stick. What counts as a war crime is currently governed primarily by the Geneva Conventions and similar laws; and they are, I think, much, much narrower than many people assume. War crimes, for instance, are committed by individuals, and are distinct from rogue or hostile actions by states; if you cannot (in principle) identify a particular individual responsible for the action, you do not have enough to establish that a war crime has been committed. Other actions could still be unjust or a violation of international law, of course; but war crimes are very specific kinds of injustice and very specific kinds of violation of international law.
In talking about these matters, there is inevitably a 'badness inflation', in which people talk about all actions with bad consequences as if they were injustices and all injustices as if they were war crimes; but this is purely rhetorical, and we should not lose sight of the actual distinctions. There is, I think, an incredible naivete, particularly among academics, about how ethical principles operate in complicated situations; people foolishly try to oversimplify the matters involved in order to jump as quickly as possible to an answer. But not every tragedy is an injustice, and not every injustice is a crime. Conflating them simultaneously inflates the rhetoric used in discussing the matter and empties our more serious vocabulary of substance. If you start calling everything a war crime, then 'war crime' becomes a mostly unenforceable accusation put on the table with no function but to try to manipulate people; it stops being a useful term for dealing with particular kinds of injustice committed by individuals that are actually punishable in the right circumstances.
