Every society that has fought wars has also argued about them, and behind those arguments sits one hard question: can the deliberate killing of human beings in war ever be morally justified, and if it can, what limits apply? Three broad answers have competed for more than two thousand years, and each deserves a hearing from its strongest defenders before we turn to the one that most governments, armies and courts now claim to follow.

Realism: war beyond morality

In 52 BCE the Roman orator Cicero, defending a client accused of murder, used a phrase that has outlived its occasion: "laws are silent in the midst of arms." Taken as a theory, this is the first answer. The realist holds that moral judgment has no real grip on war. States fight from necessity, fear and interest. Talk of justice, on this view, is either naive or a mask that the powerful wear to make their wars look noble.

Realism has a serious case. It is honest about what armies actually do, and it warns against a real danger. Realists argue that wars fought for moral causes tend to become crusades without limits, because an enemy cast as evil cannot be bargained with and must be destroyed. A state that fights only for interests can stop when its interests are secured.

The criticism is that realists cannot stop using moral words themselves. They speak of cowardice, treachery, massacre and atrocity, and those are moral judgments. If morality truly fell silent in war, none of that talk would make sense. Michael Walzer opened his influential 1977 book on the ethics of war with roughly this argument: the way people actually speak about war shows that they take moral judgment of it for granted.

Pacifism: never

At the opposite pole stands pacifism. The Dutch scholar Erasmus gave it a famous slogan in an essay of 1515 built on an old proverb, Dulce bellum inexpertis: war is sweet to those who have not tried it. Leo Tolstoy, in The Kingdom of God Is Within You (1894), argued that the command not to resist evil by violence ruled out all war. Religious communities such as the Mennonites, the Quakers and the Brethren have carried this conviction for centuries, often paying for it with prison or exile.

Pacifism comes in more than one form. Absolute pacifism rejects all killing in war, even in defense. Its defenders argue that deliberately killing a person treats a being of infinite worth as an obstacle, and that the harms of war always outrun the intentions of those who start it. Contingent pacifism accepts that a just war is possible in principle but doubts that any real war meets the test.

The standard objection is that absolute pacifism may leave the innocent undefended against those who will kill them anyway. Pacifists answer that refusing to kill is not refusing to resist: strikes, boycotts, noncooperation and protection by unarmed witnesses are forms of struggle too, and they carry far fewer risks of making things worse.

The just war tradition: sometimes, within limits

Between these poles lies the third answer. The just war tradition holds that war can sometimes be justified, but only for the right reasons and only when fought within limits. It asks two separate sets of questions. Jus ad bellum concerns the justice of going to war: is there a good enough reason, and has the decision been made rightly? Jus in bello concerns the justice of conduct during war: whom may soldiers attack, with what weapons, and at what cost to bystanders? A third set, jus post bellum, asks about the justice of how wars end.

Critics attack from both sides. Realists say the tradition ties the hands of those who must defend their countries. Pacifists say it hands every warmaker a respectable vocabulary, since leaders always claim their wars are just. Defenders reply that a shared vocabulary is precisely what lets citizens test such claims, and that a standard which is sometimes abused is still better than none.

Worked example: Sherman at Atlanta

In September 1864, after capturing Atlanta, the Union general William Tecumseh Sherman ordered its civilian population to leave the city. The mayor and two councilmen wrote to him, pleading for the old, the sick and women close to giving birth. Sherman replied on 12 September: "War is cruelty, and you cannot refine it; and those who brought war into our country deserve all the curses and maledictions a people can pour out."

Each of the three answers hears that letter differently. A realist hears an honest soldier refusing to pretend. A pacifist agrees that war is cruelty and concludes that it should not be fought at all. A just war thinker can grant that the Union's cause, preserving the country and by then ending slavery, may well have been just, and still reject Sherman's inference. Whether a war is just and whether a particular act within it is just are two different questions. A good cause does not license every means, and the people of Atlanta had not themselves chosen the war. Keeping those questions apart is the habit this course will try to build.