It's been awhile so it seems again it's time for a helpful reminder that noncombatant immunity isn't just a good idea, it's the law.
In other words: You're not allowed to kill civilians.
Killing civilians is against the law. Killing civilians makes you a criminal.
Yes, but …
No buts about it. You're not allowed to kill civilians.
And, also: You're not allowed to kill civilians.
This is neither new nor controversial, yet putting the matter in such stark terms always seems to upset people.
On the one hand, this isn't surprising since the killing of civilians has become a scarcely remarkable, dog-bites-man commonplace. Yet it's still surprising that anyone could find this elementary notion upsetting: You're not allowed to kill civilians. If you're one of those people who finds this upsetting, bear in mind what it is that you're upset about. Apparently someone you feel ought to be immune from criticism has been killing civilians and you feel I'm criticizing them by pointing out — in the most abstract terms, without any mention of particulars — that this is something that no one is allowed to do.
"What you really mean …" people say — because they're certain that when I say "You're not allowed to kill civilians" I must really mean something other than "You're not allowed to kill civilians" — "What you really mean is that you're not not allowed to target civilians."
What I really mean — and again it's not just me, or my opinion, or my preference, it's the law — is that You're not allowed to kill civilians.
"But what if …?" And here come the hypotheticals (which aren't really necessary since the world is full of actuals) positing all the many scenarios in which it is not only acceptable, but obligatory, to take some action that will, in fact, result in civilians getting killed.
Military officers really can, do and must think in such terms. That's what separates an army from a barbarian horde. That's what separates a soldier from a thug with a gun.
The key elements here are the intent, the justice/goodness and necessity of the primary effect, and the inescapable/unavoidable nature of the secondary, unintended effect. All of which sets the bar considerably higher than the oversimplified cartoon version of "the ends justifies the means."
If there is any possible way to achieve the intended effect without producing the unintended effect, then double-effect does not apply — the doctor may not slice, the general may not attack. If there is any possible way to achieve the necessary intended effect without producing the unintended effect and you act, instead, in a way that produces this secondary effect, then you have not produced "collateral damage," you have simply slaughtered civilians.
And, by the way, You're not allowed to kill civilians.