The just war theory (Latin: bellum iustum) is a doctrine, also referred to as a tradition, of military ethics that aims to ensure that a war is morally justifiable through a series of criteria, all of which must be met for a war to be considered just. It has been studied by military leaders, theologians, ethicists and policymakers. The criteria are split into two groups: jus ad bellum ("right to go to war") and jus in bello ("right conduct in war"). There have been calls for the inclusion of a third category of just war theory (jus post bellum) dealing with the morality of post-war settlement and reconstruction. The just war theory postulates that war, while it is terrible but less so with the right conduct, is not always the worst option, but justifiable when justice is an objective of armed conflict. Important responsibilities, undesirable outcomes, or preventable atrocities may justify war.
Opponents of the just war theory may either be inclined to a stricter pacifist standard (proposing that there has never been nor can there ever be a justifiable basis for war) or they may be inclined toward a more permissive nationalist standard (proposing that a war need only to serve a nation's interests to be justifiable). In many cases, philosophers state that individuals do not need to be plagued by a guilty conscience if they are required to fight. A few philosophers ennoble the virtues of the soldier while they also declare their apprehensions for war itself. A few, such as Rousseau, argue for insurrection against oppressive rule.
