A war fought for a just cause can still be fought unjustly. The rules of jus in bello apply to both sides equally, whatever the rights and wrongs of the war itself. Three principles carry most of the weight: distinction, proportionality and necessity.

Distinction

The principle of distinction says to attack only combatants and military objectives, never civilians on purpose. Who counts as a combatant? The tradition's answer is subtle. Combatant status depends on a person's role in the fighting, not on personal guilt. A frightened conscript who believes nothing his government says may be attacked; a cabinet minister who planned the war may not be attacked while he is at home, at least by soldiers, though he may later face trial. A worker in a munitions factory may be killed in an attack on the factory, but not hunted down in her house after her shift.

Memoirs also record soldiers refusing shots the rules allowed. George Orwell, fighting in Spain in 1937, recalled that he could not fire at an enemy running half-dressed and holding up his trousers, because the man looked simply like a fellow human being.

Proportionality and necessity

Many legitimate attacks will also harm civilians nearby. The law, as set out in a 1977 protocol to the Geneva Conventions, answers with a balancing test. Proportionality in war requires that expected civilian harm not be excessive compared with the concrete military advantage expected. This is not the same test as the one in level three. Proportionality in war judges individual attacks, while proportionality for going to war weighs the war as a whole. A justified war can include disproportionate attacks, and an unjust war can include attacks that are proportionate taken one by one.

The principle of necessity says that force may be used only as far as a legitimate military aim requires. Necessity limits violence, but it has also been used to excuse it. In the late nineteenth and early twentieth centuries some German military writers championed a doctrine called Kriegsraison. The doctrine of Kriegsraison claimed that military necessity could override the laws of war. After the Second World War, tribunals rejected it: the laws of war already take necessity into account and cannot be set aside whenever a commander finds them inconvenient.

Intending and foreseeing

The tradition has long distinguished between harm a soldier aims at and harm he foresees but does not aim at. A pilot who bombs a bridge knowing that a nearby house may be hit is not in the same moral position as one who targets the house to spread terror. Walzer accepted this distinction but thought it too easy on soldiers. In his revision, sometimes called double intention, soldiers must not only avoid aiming at civilians but actively try to reduce harm to them, even at some risk to themselves. A unit that could warn villagers before an attack, or approach a target more slowly and carefully, is obliged to do so.

Some philosophers insist on firm limits that no calculation can cross. In 1956 the Oxford philosopher Elizabeth Anscombe publicly opposed the university's decision to give an honorary degree to former US President Harry Truman. Anscombe argued that choosing to kill the innocent as a means to one's ends is murder, whatever good results are claimed for it, and that Truman's order to use atomic bombs on cities was a choice of that kind. She lost the vote, but her argument is still a touchstone for absolutist views.

Supreme emergency

Walzer himself allowed one exception. Under the idea of supreme emergency, rules against attacking civilians may be overridden only when a community faces imminent and catastrophic danger. His example was Britain in 1940 and 1941, alone against a Nazi Germany whose victory would have meant enslavement and mass murder across Europe. He argued that bombing German cities might have been permissible then, as a last desperate measure, but not later, when the danger had passed and the bombing continued. Critics on one side say any such exception will be stretched by every desperate government. Critics on the other side, like Anscombe, say that murder does not become innocent because one's situation is desperate.

Worked example: the Amiriyah shelter

Before dawn on 13 February 1991, during the Gulf War, two American laser-guided bombs struck a reinforced concrete bunker in the Amiriyah district of Baghdad. More than four hundred civilians, many of them women and children, had been sleeping inside. American officials said intelligence showed the bunker was being used as a military command post. Iraqi officials said it was a civilian shelter.

Work through the principles. If the bunker was a command post, it was a military objective, and distinction did not forbid attacking it. The civilian deaths were not the aim, so the line between intending and foreseeing applies. But proportionality and double intention ask harder questions. What did planners know about civilians using the building at night? Could they have watched it longer, struck at another time or found another way to disrupt the command post? Afterward, according to many accounts, strikes on Baghdad were sharply reduced and individually reviewed at a higher level.