Children are dying in Gaza. In the coming days, more will die. And though many die as shields cruelly used by cynical Hamas terrorists, they are being killed by bombs from Israeli planes and shells from Israeli tanks. And so even as we acknowledge that Hamas’ hands are stained with the blood of its own people, for Israel, too, there must be a moral accounting.

I do not mean simply a refrain we have heard frequently in recent weeks: “No country on earth would tolerate missiles raining down on its citizens.” This sentence is probably true. But it is also irrelevant. The question is not what some other country would do, but what Israel ought to do. And that question is not as easily answered. In fact, it presents us with one of the great moral paradoxes and tragedies of our time: A war which must be fought—and which seems impossible to fight morally.

Traditionally, moral thinking about war is divided into two broad questions. First, we ask whether the decision to go to war was a moral one. In doing so, we ask: Are the reasons for the war morally compelling? Were less-destructive alternatives considered and pursued?

For Israel, the first question seems easier to answer. Few would deny that, in principle, Israel’s war with Hamas is both just and necessary. Israel acts on the most clear justification possible: self-defense after days of restraint, warnings, and pleas—as rockets continued to land on its cities and later, as militants sprang from tunnels to kill its citizens. Ceasefires have been offered, but Hamas has rejected them. And whatever criticisms one may have of Israel’s failures to midwife an effective and peaceful alternative to Hamas (and I have many), these do not undermine the fundamental justice of this self-defense.

But there is also a second, larger question: How should wars be fought? And here, Israel runs into a problem. Because in the conduct of war, we insist not only that combatants be the sole targets of military action or that steps be taken to reduce civilian deaths. But we also insist on proportionality; that the military value of a target must outweigh the anticipated harm to civilians.