While Catholic moral theology forbids absolutely the direct or intentional killing of the innocent, it does not prohibit all actions that result in the deaths of innocents. Under the doctrine of the double effect (also known as the principle of side effects), an action resulting in the deaths of innocents may nevertheless be justifiable so long as those deaths are not intended either as a means or as an end, and as long as the good that results from this action outweighs the bad. For this reason, Catholic moralists have typically said that some level of collateral damage (that is, unintentional killing of the innocent) can be permissible in war.
Exactly how much collateral damage can be tolerated in a military action is, of course, no easy question. Judging the consequences of an action means speculating about the future, something we humans are not terribly good at. One cannot give a set number or ratio below which civilian casualties in a military operation are acceptable and above which it is not, as too much depends on the particular circumstances of the individual case. Certainly I thank God that I am not in the position of having to make such decisions, weighing the near certainly of a small number of civilian casualties against the probability or possibility of a much greater number of deaths. Nevertheless, as I consider things like the incident yesterday in Somalia, my mind cannot help but return to the story in Genesis of Abraham pleading with God to spare the innocent of Sodom and Gomorrah:
Then the LORD said: “The outcry against Sodom and Gomorrah is so great, and their sin so grave, that I must go down and see whether or not their actions fully correspond to the cry against them that comes to me. I mean to find out”….
Then Abraham drew nearer to him and said: “Will you sweep away the innocent with the guilty? Suppose there were fifty innocent people in the city; would you wipe out the place, rather than spare it for the sake of the fifty innocent people within it? Far be it from you to do such a thing, to make the innocent die with the guilty, so that the innocent and the guilty would be treated alike! Should not the judge of all the world act with justice?”The LORD replied, “If I find fifty innocent people in the city of Sodom, I will spare the whole place for their sake.”
Abraham spoke up again: “See how I am presuming to speak to my Lord, though I am but dust and ashes!What if there are five less than fifty innocent people? Will you destroy the whole city because of those five?”
“I will not destroy it,” he answered, “if I find forty-five there.”
But Abraham persisted, saying, “What if only forty are found there?”
He replied, “I will forebear doing it for the sake of the forty.”
Then he said, “Let not my Lord grow impatient if I go on. What if only thirty are found there?”
He replied, “I will forebear doing it if I can find but thirty there.”
Still he went on, “Since I have thus dared to speak to my Lord, what if there are no more than twenty?”
“I will not destroy it,” he answered, “for the sake of the twenty.”
But he still persisted: “Please, let not my Lord grow angry if I speak up this last time. What if there are at least ten there?”
“For the sake of those ten,” he replied, “I will not destroy it.” – Genesis 18:20-32.