https://www.open.edu/openlearn/history-the-arts/the-moral-equality-combatants/content-section-1
Three components of the Just War Tradition
In this course we are going to be thinking about war, and the morality of war. To understand what philosophers have written in this area, we need to distinguish two things. The first is a tradition of thought, and the second an area of philosophical enquiry.
The first, known as โthe Just War Tradition โ is a long historical tradition which is difficult to summarise in a non-controversial manner. It is, more or less, the claim that there is a set of conditions which can act as a kind of checklist for whether a war is just or not. The Just War Tradition takes its form because it originated as a source of advice for princes and kings who were considering whether or not to wage war. In particular, the early modern just war theorists were advising Christian princes on whether their warfare was justified. They were advised that, if their actions met the conditions specified the Just War Tradition, then they were morally justified in waging war. One reason why the Just War Tradition is important today is that it has been encoded in the rules of war โ the laws covering international conflict โ at the Geneva Convention, the Hague Convention and so on. These conditions were conventionally divided into two groups, one concerned with when one may go to war and the other concerned with how one may fight. These two sets of conditions are standardly known by their Latin tags: Jus ad Bellum and Jus in Bello