War is a matter of “it’s us or them.” Leaving the option of surrender to conquest out of consideration, each side must decide whether it is to be the one to perish. The horror of war is what flows from each side’s taking the obvious decision to fight: vast numbers of killings, maiming, starvation, and other attendant miseries. Borrowing from Hobbes’s description, we can say that war is “nasty” because it relentlessly poses the question of whether one will choose to perish or choose to fight dirty. By “dirty,” I do not mean inflicting more than the amount of harm necessary to repel the enemy. Rather, I mean doing what is ordinarily done in war: horrific quantities of slaughter of innocents as a necessary means of repelling the opponent.
So, war being dirty and horrible, a brutal principle W makes itself distastefully evident whenever the justice of this horror is contemplated:
W: If side A is justified in waging war against side B, then A is justified in slaughtering vast numbers of B combatants and non-combatants, to the extent that A reasonably considers this to be necessary to conquer B without incurring vastly many more casualties than A will by committing the slaughter.In other words, if A is justified in waging war against B, then A is also justified in killing many enemy innocents, as long as this is necessary to winning the war. This is why the bombings of Dresden, Tokyo, Hiroshima, and Nagasaki were justified horrors. W goes to the essence of war’s horror. War is peculiarly horrible precisely because there is no way of avoiding the perpetration of its horrors via moral reasoning. Moral reasoning bids one commit them. Unlike the Portuguese earthquake of 1755, for instance, war is a horror which morality requires that one intentionally bring about. The horror is not just in the death and suffering. It is also in the fact that morality is this way.
The justification for W is Hobbesian. A nation is not morally obligated to let itself be slaughtered by an enemy just because repelling the enemy’s attacks will cause the deaths of enemy innocents. If one has a right to defend oneself against an enemy during war, the prospect of enemy innocents doesn’t override that right. W follows from this premise. As for the premise, I take it to be non-controversial. There are pacifists who might reject it, but pacifism isn’t plausible enough to be worth arguing over. The premise follows from the fact that pacifism is false.
There are more topics here. Who is responsible for the slaughter of innocents in war according to principle W? Does principle W make any distinction between collateral damage and intentional, strategic slaughter of innocents? Let’s address those next.