Just War Theory (Latin: jus bellum iustum) is an ancient tradition of leading a war or participating in it. It includes the ethical and moral vision of the issue and has a set of rules defining whether a country can (should) start a war, or, if attacked, whether it can (should) participate. The just war theory used to be a matter of study for military leaders, ethicists, theologians, and policymakers. It might also be called military ethics.
Those previously mentioned rules and requirements could be divided into two categories, first of which is “right to go to war” (jus ad bellum) and “right to conduct in war” (jus in bello). The most recent idea was adding the third category to the list, which would be “dealing with the morality of post-war settlement reconstruction” (jus post bellum).
This theory states, that war, though being awful and terrible isn’t the worst choice sometimes. Some things might justify war, like undesirable outcomes, preventable atrocities or essential responsibilities.
There are the opponents of the theory, of course. They can be divided into the loads of the groups, but two main opponents are pacifists, of course, as they never see reasons, that could justify war, and different kinds of nationalists movements, which state, that the war should, first of all, serve the nation’s interests, and that is the only way for it to be justified.
If we look at it in the historical perspective, the tradition of leading a “just war” refers to the body of specific rules and agreements, which were applied in numerous wars across the ages. The tradition also includes loads of written works of various philosophers, lawyers and ethicists, who tried to examine or evaluate the vision of the ethical boundaries of war and whether their thoughts were a solid contribution to the set of conventions, which rule the conflicts and warfare.