No government will send young men into battle to kill and be killed without offering some justification for what they are doing, Walzer says.

Question: What is Just War Theory?

Michael Walzer:  Yes, well, first of all, it’s a very old theory because no government in any high civilization, and in many low civilizations, no government will send young men into battle to kill and be killed without offering some justification for what they are doing. And so Just War Theory is simply an argument about what justifications make sense. What are the plausible justifications and how do we as citizens judge what governments do when they go to war? And the Greeks had arguments about when to fight and how to fight and there are biblical accounts of arguments about when to fight and how to fight, and in the Islamic tradition there are arguments. Just War Theory, as a doctrine, it comes out of catholic moral theology in the Middle Ages. So this is an ongoing and a very long ongoing argument and in---among us the argument has a double character. There are arguments about when to go to war, when is it justified. This is called jus ad bellum, the justice of war, and we hang on issues of aggression it is right to resist there an attack, just as you could resist on the city street if you were mug you could fight back and that would be adjust miniature version of a just war. And it’s also just to come to the aid of the victim of aggression, as you might do on a city street if you were brave enough, and that would be a just war. And we have also begun to think that when there is a massacre going on inside a state, when a government is massacring some minority or maybe even not a minority, a majority of its citizens, the way the Cambodian Khmer Rouge regime did, that it is just to go in and stop it by force if necessary. So those of the just occasions of war and then there is justice in war, jus in bello, justice in the conduct of war, and that hangs mostly on issues of non-combatant immunity, of discrimination, of attacking only other soldiers, so it hangs on a very old idea that war is a combat between combatants from which non-combatants should be shielded. Non-combatants means women, children, old men. It means medical personnel. It means religious officials. It has even meant, in some circumstances, the merchants who sell weapons to both sides were in some accounts treated as non-combatants who nobody should attack, but basically for us it means the civilian population, is not, should not be, subject to attack in the war.