War is monumentally destructive, deeply tragic, and, to many, morally incomprehensible. War involves death on an awful scale. In the war between Vietnam and the United States (1960–1975), an estimated 2.3 million people died. In warfare in the Democratic Republic of the Congo between 1998 and 2007, an estimated 5.4 million people died. In World War II (1939–1945) an estimated 70 million people died. In the many wars that occurred in the period from the end of World War II until 2000, an estimated 41 million people died. It is claimed that in the 3,500 years of recorded history, there have been only 270 years of peace, and that the United States has enjoyed only 20 years of peace since its founding. In war, people suffer and die in appalling numbers and in appalling ways. It says something important and terrible about humans that we are capable of engaging in such destructive activities. In the face of such devastation, what sense does it make to talk about the ethics of war? Isn’t the very phrase an oxymoron? War seems to be a moral outrage, not to be tolerated. Yet people do talk about war in moral terms. They distinguish between the morally acceptable and the morally unacceptable in wars and ways of fighting. There are deep relations between war and morality, despite any initial appearance to the contrary, and it is these relations we will explore in this book. Despite the moral horror that war can be, it is sometimes the morally preferable choice.

Rwanda, 1994

From April to June of 1994, 500,000 to 1 million citizens of the Central African state of Rwanda were slaughtered by their compatriots. The victims were primarily members of the Tutsi ethnic group, and the murderers members of the Hutu ethnic group. The Hutu perpetrators set out to destroy the Tutsis. It was a clear case of genocide, the worse such case since the Holocaust of World War II. Genocide is a systematic effort to destroy “a national, ethnical, racial or religious group, as such.” In a genocide, a large number of people, men and women, young and old, are killed for no reason other than that they belong to the group into which they were born. Genocide is perhaps the worst moral act humans can commit, and attempts to stop it from happening are, correspondingly, morally imperative.