The only allowable situation for causing the death of a human being, then, is self defense. You will not find a justification of pre-emption anywhere in the Catechism that allows you to kill a merely potential aggressor.
What is lawful, however, would be to foreclose the possibility that Aunty Ani will become an aggressor. The most effective way to do this, and to guarantee not only that Aunty Ani, but all the other workers at the bomb factory, do not become aggressors, would be to remove the means of their aggression. In other words, get rid of the factory, or otherwise render it no longer a threat.
Our agent could, instead of breaking into poor Aunty Ani’s house and murdering her in her sleep, instead proceed down the road to the factory and sabotage all the machinery and destroy all the stores. If we had enough agents on the ground, they could do this to each and every factory.
Of course, this is hardly feasible, so even though this respects the workers, who while sleeping are not workers but rather human beings first and foremost, it does fail on the third point of just war theory “there must be serious prospects of success” (CCC 2309).
This probably eliminates a paratrooper invasion and a naval bombardment as well, leaving us with aerial bombardment. The same considerations that guided our thinking of what is acceptable for our agent on the ground, however, apply when considering aerial bombing, for the moral law applies even when (I would say especially when) we are at war (2312). In other words, if it was immoral to target Aunty Ani for death when our agent was on the ground, it remains immoral to target Aunty Ani for death when planning our bombing sortie.
The factory Aunty Ani works in would thus be a legitimate target, but her house would not be. This effects our choice of weapon and our tactic, naturally. Carpet bombing would not be justified, because in that case how could we say that we are not targeting Aunty Ani’s house? No, in that case we would be targeting everything, without discriminating between combatants and non-combatants, legitimate and illegitimate targets. For the same reason, a weapon that effectively has as its target the entire city (i.e. a nuclear bomb) is out. This leave us with conventional, targeted bombs.
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Ah, but you say, conventional bombs in WWII were notoriously inaccurate. True, but to say that they had only rudimentary target control is not the same as having *no *target control. The commander ordering the strike against a series of legitimate targets embedded within non-legitimate targets, as was the case in Hiroshima and Nagasaki, has a positive duty to use the most accurate and reliable weapons available. He must make a good faith effort to target legitimate military objectives and avoid non-combatants.
Assuming he chooses the most accurate weapons and delivery mechanisms available to him, then, do we not still run into the problem that civilians will die? Yes, but remember the claim was not that a nuclear bomb or carpet bombing technique is wrong because civilians die, it is wrong because it would fail to discriminate between combatants and non-combatants. For instance, if we assume that the factories are running 24/7 and that the entire population of the city remains within the walls of the factories at all times, then destroying those factories would have the same effect, in terms of loss of life, as a nuclear bomb. The difference, of course, is that the circumstances are quite different – because they are in the factory, they are at that moment acting as active aggressors. If they were to spot the bomber coming and flee from the factory, the bomber could continue on its mission to destroy the factory, but could not then attempt to strafe the fleeing workers (who, with their factory destroyed, are no longer workers at this point but rather human beings running for their lives).
Similarly, should a bomb go astray (as they often did) and land hit a house instead of the factory, killing all within, the intent was not to hit that house. Of course, if the bomb hit the house because the crew of the bomber were too lazy to aim, or because the bomb manufacturer cut costs and provided a substandard product, the person or persons responsible bear moral guilt for that death. Should the bomb simply go despite the best-faith efforts of all involved to produce a bomb that was as accurate as possible and target a legitimate military objective, though, there is no moral culpability.