There are two different types of just-war criteria. First, there is criteria for going to war. What conditions are necessary to enter a war righteously? Second, there is the criteria for conducting a just war. What is righteous when waging war?  It is possible to violate the first,  in other words go to war for an unjust reason, but conduct the war in a just manner. There are three criteria that just-war thinkers use to determine whether or not going to war is just. Here is the first. This entire quote is from Charles and Demy’s book War, Peace, and Christianity. 

Just Cause. To establish the justness of a cause is to make fundamental moral distinctions-for example between innocence and guilt, between the criminal and punitive act, between retribution and revenge, between egregious human-rights violations (“crimes against humanity”) and the need for humanitarian intervention to restore basic human rights. In principle, just cause is motivated by two chief concerns: to rectify injustice or to prevent injustice; hence Aquinas can argue that “those who are attacked are attacked because they deserve on account of some wrong they have done.”

As fundamental as this idea is to Christianity and to humanity, in our age it difficult to swallow. Why? Many people, even Christians, are not sure objective truth can even be discovered. Can we really tell who is right and who is wrong? Aren’t all sides wrong in a war?  Aren’t we just talking about degrees of guilt with no real innocence? Who can even determine if a cause is just or not? Relativity has infected us to such a degree that many Christians not only cannot tell the difference between innocence and guilt, they are not sure such a difference even exists, at least in the practical area of day to day life.

As I read Charles and Demy’s book one thought that struck me over and over again was that just-war thinking only works in a world of objective truth. If we cannot know what is true and good then justice is lost. For just-war thinking to prevail there must be a standard of guilt and innocence and the belief that with time and wisdom we can determine who the guilty party is in a conflict.