If a State A attacks another State B's military apparatus knowing full well that there will be civilian collateral damage, then why is it that even if State B retaliates by intentionally targeting civilians, it's terrorism?

State A may know that there may be harm caused to civilians, as a matter of probability, but without targeting civilians, for example during the invasion of Normandy. If State B targets civilians and does so with the intention of causing terror, that is a very different matter, for example in the Blitz and the bombing of Dresden. At least so says the just war theory. And there is sense to it. A defensive war could not be fought if one expected no unexpected harm to innocents. But intentionally and deliberately harming civilians or non-combatants is an element in terrorism and more broadly in military actions that are not just. All this (and more) is the point of the law of double effect, which is an extension of Aquinas' treatment of homicidal self-defense. In defence of myself, do I intend to kill my assailant? Or does that killing come about as a foreseeable but unintended consequence of my self-defense? Besides, Aquinas' example was concerned only with self-defense, and there is no imaginable interpretation on which the bombing of London (apart from the docks, perhaps) could be construed as national self-defense. This is not quite true of the fire-bombing of Dresden, though the city had no military significance; but that and other particular examples belong to the historian more than the philosopher.

A brief addition to Jonathan's fine response. Of course different people define "terrorism" differently, but it looks like you're using the word to mean "intentional targeting of civilians" or something like that (independent of what the motive is for targeting the civilians). One (of many) ways to think about the difference is this. What's wrong isn't the "killing of civilians" per se, it's the "killing of civilians for no (or beyond any) legitimate military purpose." If we then permit the killing of civilians for bona fide military purposes (e.g. as collateral damage of bona fide military attacks) but forbid the killing when it lacks such purposes, then, overall, long term, over all conflicts (assuming combatants all follow these moral rules!), there are likely to be far fewer civilian casualties than if we treat the two cases identically. Put differently, if you could kill civilians at will, then both sides will kill lots of civilians; but if you can only kill civilians as collateral damage, then your attacks will be restricted and thus likely to produce fewer overall civilian casualties. (But clearly lots more to say here.)

best,

ap

Andrew is obviously right, but what he is proposing is actually a utilitarian basis for double effect.

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