In my last column, I argued that we cannot suspend the moral requirement to love our neighbors in wartime. Even the enemy soldiers whom the Christian must reluctantly kill are precious in God's sight. The column provoked such an interesting range of e-mails (pro and con) that I want to further pursue the subject of war in Christian thought.

In an essay entitled "War and Massacre," philosopher Thomas Nagel asks us to envision a conversation between two soldiers on the battlefield. Each seeks to explain to the other why he must be killed. This scenario reminds us that we must always justify so serious a moral act as killing. We cannot do it until we are persuaded, morally, that it is necessary.

The criterion of necessity is a difficult one, and the early Christians struggled with it. They knew that Christians must not kill casually. Many believed that Christians may not ever kill. Today, most Christians—like most other people—accept killing (even if reluctantly) when grim necessity forces upon us a war we would rather avoid.

For example, most of us agree that war is morally permissible to defend one's own country against aggression. There are difficult questions still. (What counts as aggression? May one attack before the enemy strikes?) But wide consensus exists on the general principle of self-defense.

The more difficult question, and one that increasingly confronts the world, is the justice of going to war to protect not our own people but someone else's. In other words, if Christian morality will permit Country A to fight Country B if Country B attacks it, will it allow Country A to fight Country B if Country B attacks Country C? Or what if Country B is slaughtering only its own people? May Country A go to war to protect the ...